ORIGINAL PAPER

Deadly CO2 gases in the Plutonium of Hierapolis (Denizli, Turkey)

Hardy Pfanz1 & Galip Yüce2& Ahmet H. Gulbay3 & Ali Gokgoz4

Received: 30 March 2016 /Accepted: 16 January 2018 /Published online: 12 February 2018# Springer-Verlag GmbH Germany, part of Springer Nature 2018

AbstractUsing a portable gas analyzer system, the geogenic gas regime below and around an ancient gate to hell at Hierapolis/Phrygia wascharacterized. The site was first described by Strabo and Plinius as a gate to the underworld. During centuries, it attracted evenancient tourists. In a grotto below the temple of Pluto, CO2 was found to be at deadly concentrations of up to 91%. Astonishingly,these vapors are still emitted in concentrations that nowadays kill insects, birds, and mammals. The concentrations of CO2

escaping from the mouth of the grotto to the outside atmosphere are still in the range of 4–53% CO2 depending on the heightabove ground level. They reach concentrations during the night that would easily kill even a human being within a minute. Theseemissions are thought to reflect the Hadean breath and/or the breath of the hellhound Kerberos guarding the entrance to hell. Theorigin of the geogenic CO2 is the still active seismic structure that crosses the old town of ancient Hierapolis as part of theBabadag fracture zone. Our measurements confirm the presence of geogenic CO2 in concentrations that explain ancient stories ofkilled bulls, rams, and songbirds during religious ceremonies. They also strongly corroborate that at least in the case ofHierapolis, ancient writers like Strabo or Plinius described a mystic phenomenon very exactly without much exaggeration.Two thousand years ago, only supernatural forces could explain these phenomena from Hadean depths whereas nowadays,modern techniques hint to the well-known phenomenon of geogenic CO2 degassing having mantle components with relativelyhigher helium and radon concentrations.

Keywords Carbondioxide .Helium .Radon .Charonion .Gate tohell .Geogenicgases .Hades .Mephitic exhalations .Mofette

Introduction

The Pamukkale thermal province located in the east of theBuyuk Menderes Graben is famous for its thermal springsand snow-white travertine formations and also for the excava-tions of ancient Hierapolis. First recordings of the old city ofHierapolis were made by Strabo (XII, 8, 17) and also Pliniusthe Elder (Nat. Hist. V, 105) mentions this famous town. The

town, probably a Seleucid foundation in the second centuryB.C. (Gerster 2005 p. 162; Porter 2016 p. 355), developedduring the Roman Empire and was already famous inByzantine times (Ring et al. 1995 p. 327; Rigsby 1996;Cohen 2006; Zwingmann 2012; Nyquist 2014; Şimşek andD’Andrea 2017). In this period, a huge pilgrim sanctuary wasbuilt around the tomb of the Apostle Philip (D’Andria 2003).Geologically, the holy town Hierapolis is cut longitudinally byseveral parallel fractures of the Pamukkale fault (intra-platetectonics) as the part of extensional straining of the westernAnatolian extensional regime or the west Anatolian horst-graben system extending from the Aegean Sea to centralAnatolia (Seyitoglu and Scott 1996; Kaymakci 2006). It isspecified mainly by major horsts and grabens at E–W trending,and NW–SE to NE–SW oriented relatively short and locallysuspended cross-grabens (Bozkurt 2003). Thus, Hierapolis issituated in one of the most tectonically active regions of AsiaMinor. Therefore, the area was destroyed several times bymany earthquakes (Altunel and Barka 1996; Hancock andAltunel 1997; Altunel and Karabacak 2005; Kaymakci 2006;Uysal et al. 2007, 2009; Kele et al. 2011; Hancer 2013; De

* Hardy Pfanzhardy.pfanz@uni-due.de

1 Institute of Applied Botany and Volcano Biology, University ofDuisburg-Essen, 45117 Essen, Germany

2 Department of Geological Engineering, Hacettepe University,Beytepe, 06800 Ankara, Turkey

3 Department of Geological Engineering, Eskisehir OsmangaziUniversity, 26480 Meselik, Eskisehir, Turkey

4 Department of Geological Engineering, Pamukkale University,Denizli, Turkey

Archaeological and Anthropological Sciences (2019) 11:1359–1371https://doi.org/10.1007/s12520-018-0599-5

Filippis et al. 2012; Kumsar et al. 2016). Directly build uponsuch a fault are two interesting buildings: the famous Apollotemple and the newly discovered Plutonium, the sanctuary ofthe Gods of the Underworld, Pluto and Kore, with the theaterabove a grotto. The latter was excavated between 2011 and2013 during the campaign of the Italian ArchaeologicalMission in Hierapolis (D’Andria 2013). To be exact, this sitehas been mentioned by several antique sources as the entranceto the Underworld (Zwingmann 2012); they describe thatstrange things happened at this Hadean outlet which they calledPlutonium (Ploutonium) or Charonium. Deadly vapors weredescribed escaping from these places (Strabo XIII, 4, 14;Bejor 1984). Priests were demonstrating their supernaturalpower and their equality to the gods by ushering animals likegoats and bulls into the grotto (Plutonium) where, after a shorttime, the animals showed signs of suffocation, finally dyingafter several minutes. Yet, the castrated priests (Galli) survived(Strabo XIII, 4, 14; Plinius, Nat. Hist. II, 207–208; for reviews,see Zwingmann 2012 or Pfanz et al. 2014).

Many similar sacred Greek and Roman places, oracles, andtemples were often located directly above or close to geolog-ical disturbance zones (Piccardi 2000; De Boer et al. 2001;Etiope et al. 2006; Foster and Lehoux 2007; Piccardi et al.2008; Mariolakos et al. 2010). The famous oracle of Delphiwas built on the intersection of two fracture zones where eithermethane/ethylene venting or hypoxic aeration due to methaneor CO2 emission enabled Pythia to give her prophecies (DeBoer et al. 2001; Etiope et al. 2006; Piccardi et al. 2008).Heavy methane emissions were also found at Chimera(Etiope et al. 2006; Hosgormez et al. 2008). In all cases, eitherhigh CO2 concentrations or methane and other hydrocarbonswere emitted from geogenic sources (for a review, see Pfanzet al. 2014 and Etiope 2015).

This paper accounts for the geochemical compositions ofsoil gas of some fractures and holes around the Hierapolisfault and Hierapolis archeological sites. We concentrated oursurvey specifically on the close vicinity of the Temple ofApollo and the Plutonium of ancient Hierapolis/Phrygiawhere mystic tales and narratives of the asphyxiation of bullsduring religious ceremonies exist. We tried to (1) find and (2)quantify the noxious gas(es) inside and outside of the bothtemples and (3) examine the diurnal degassing patterns to helpunderstanding how ancient ceremonies were safely performedby the priest of the sanctuaries.

Material and methods

The location

Hierapolis is located in the Denizli graben in the southwest ofTurkey (Fig. 1a, b) which is a geological disturbance zone ex-tending between the Pamukkale and Babadag faults that are

paralleling each other (cf. Özdemir 2002; Kaymakci 2006;Hancer 2013; Kumsar et al. 2016). Several buildings ofHierapolis were directly built on the fracture zone. Two buildingsthat were already mentioned by ancient writers for their BdeadlyHadean breath^ were selected for a close determination ofgeogenic gas emissions: the Apollo temple and the newly exca-vated sanctuary of Pluto (Fig. 1c; see also D’Andria 2013).

Gas measurements

Measurements were taken during two different campaigns inMay 2013 and June 2014.The gas measurements were carriedout with the portable gas analyzer GA2000 (GeotechnicalInstruments, England; Pfanz et al. 2004) equipped with waterand dust filters. CO2, CO, CH4, H2S, and O2 were recordedsimultaneously. Problems occurred when the soil was wet, wa-terlogged, or filled with hot water vapor, as liquid water woulddamage the sensitive cells of the analyzer. Polyethylene andTeflon tubing (1 cm in diameter) was used to enlarge the mea-suring radius up to 6 m. Readings were recorded after severalseconds to one minute. Long-term measurements for CO2 weremade using a self-made CO2 analyzer, equipped with an ultra-low power (3.5 mW), non-dispersive infrared (NDIR) sensor(measurement range of 0–100%; COZIR, CO2 Meter, Florida).The warm-up time of the sensor is shorter than 10 s; operatingconditions are − 25 to 55 °C and 0–95% RH, non-condensing.The accuracy of the sensor is ± 0.5% vol. CO2, at standard tem-perature and pressure. The pressure dependence of the sensor is0.13% of the readings per millimeter of Hg.

The sensor was attached with a micro-diaphragm gas samplepump (delivery, 0.4 l/min; KNF Neuberger, Balterswil,Switzerland). The sampling time was set with a microcomputerto 30 s. At the end of each interval, the measurement value wassaved on a micro-SD card and the sensor and pump were pro-grammed to go to sleepmode until the next sampling cycle (after5 min). The battery-driven device was mounted within a water-tight hard plastic box. CO2 measurements at different heights(15, 40, 100 cm) above the ground level within the sanctuaryof Pluto were performed on different days in 2014.

In situ radon measurements were carried out by GEO-RTM2128 alpha spectroscopy instrument. A Teflon tubing wasinserted 2–3 m into the mouth hole of the Apollo temple andgrotto of the Plutonium; it was directly connected with the radonmeasurement chamber. The relative statistical error for radonactivities ranged from over 10% for low radon values (∼ 2–10 Bq l−1) to less than 5% for values greater than 10 Bq l−1.The measurement period was selected as a minimum of 1 h toobtain reliable results with a high confidence level (95%). Theisotopic composition of helium and neon was determined bymass spectrometry at INGV-Palermo. Two gas samples takendirectly from the mouth of the grotto were collected by takingcare to avoid the least amount of atmospheric gas contamination.The sampling procedure for free gases is as follows: free gases

1360 Archaeol Anthropol Sci (2019) 11:1359–1371

were collected into the pyrex bottle having a three-way valveconnected to a silicon tube which was inserted into the grotto.The silicon tube was then connected to a syringe which suckedthe gas through the valve directly into the sampling pyrex bottle.To ensure pure geogenic gas, the bottle was flushed with a gasvolume ten times larger than its own volume. For the final gascollection, the first valve was closed and by applying a slightoverpressure insight the bottle; the second valve was finallyclosed (Italiano 2009 and Italiano et al. 2013).

Chemical analyses were done by gas chromatography(Perkin Elmer Clarus 500 equipped with a double TCD-FIDdetector) using argon as the carrier gas. Uncertainties werewithin ± 5% (Italiano et al. 2013). The isotopic compositionsof helium were obtained by mass spectrometry. The isotopicanalyses of the purified helium fractions were performed by asplit flight tube static vacuum mass spectrometer(GVI5400TFT) that allows the simultaneous detection of 3Heand 4He-ion beams, thereby keeping the 3He/4He error of mea-surement to very low values. Air is routinely run as a standardfor calibration. In general, the total errors on the ratios are lessthan 2 and 5% in one sigma standard deviation, respectively

(for 3He/4He and 4He/20Ne). Uncertainties in the range of low3He (R/Ra values below 0.1) samples are within ± 5% (Italianoet al. 2013).

The 3He/4He ratios (R) have been normalized to the atmo-sphere (3He/4He = 1.39 × 10−6 =Ra) and corrected for the effectsof atmospheric contamination (R/Rac) using (R/Rac) = {(R/Ra)X − 1} / {X − 1} where X is the air and the ASW He/Ne ratio(Hilton et al. 1998).

Local weather data were obtained from the Turkish StateMeteorological Service (DMIGM) for Denizli.

Results

The new Plutonium—a deadly gas atmospherearound and within the sanctuary of Pluto and Kore

During the excavation held by D’Andria in 2011–2013, a sub-terranean grotto was found below the stone-seats of thePlutonium (Fig. 2a). The grotto belongs to the sanctuary ofPluto (Hades) and Kore (Persephone). The words BPloutoni kai

Archaeol Anthropol Sci (2019) 11:1359–1371 1361

Fig. 1 Location of the area of Hierapolis/Denizli in southwest Turkey (a).Position of the two sanctuaries, the sanctuary of Pluto (Plutonium) and theTemple of Apollo (top left) in the centrum of the Hierapolis excavatedarea. The positions of sinkholes in the close vicinity of the sanctuaries are

given (b). Known faults and grabens in the Denizli graben and its rimsincluding the towns of Hierapolis, Laodikea, and Denizli. Maps weremodified from Kumsar et al. 2016 (c)

Kore^ in Greek letters engraved into the stone row below theseats are still readable (D’Andria 2013). Due to the incompleteexcavation state of the Sanctuary in 2013, only the small, upperpart of the grotto and the antechamber was accessible for gasmeasurements (Fig. 2a). In 2013, access to the interior of thesubterranean grotto was possible only through a small mouthhole (red arrow in Fig. 2a). One year later (in 2014), the lowerpart of the Plutonium was fully excavated and freed the oldbasement (Fig. 2b). Unfortunately, hot, carbonate-rich waterpoured into this basement from the interior of the grotto. Thewhole front area below the seat rowswas thus floodedwithwaterup to a height of 10 cm. Thus, proper CO2 gas measurementswere only possible above the water phase (ca. 15 cmaboveground).

Gas data from the 2013 campaign

Due to the incomplete excavation state of the Plutoniumin 2013, gas measurements inside the closed subterranean

chamber (grotto) of the Plutonium were performed byinserting a 6-m long Teflon tubing into the small openingof the excavated wall (arrow in Fig. 2a). At a depth of2 m, maximum CO2 concentrations inside the grotto were91%. Measurements were herewith done at the mysticalplace where hellhound Kerberos is expected to have itsguard post (Bloomfield 1904). Yet, due to the smallmouth hole and the total darkness inside the grotto, theexact position of the end of the tubing could not exactlybe located. The complete basement of the grotto was to-tally dark but seemed to be warm and highly humid (dueto a warm carbonate-rich creek).

All CO2 measurements within the grotto gave similarresults, independent of the exact position of the tubing. Arather homogeneous gas lake was therefore assumed with-in the grotto with more or less constant CO2 concentra-tions ranging between 86 and 91%. As neither wind norsun could enter the closed grotto, no large changes of thegas concentrations were to be expected. Only close to the

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Fig. 2 Stone seating rows for thespectators of the ceremonies asseen in the freshly excavatedPlutonium within the Sanctuaryof Pluto, picture from 2013. Theantechamber in front (blue arrow)of the subterranean grotto (whitearrow) is seen. Also shown is thehole within the walls of the grottowhere the geogenic gas escapedfrom the interior to theatmosphere (red arrow) (a). Asimilar scene of the Plutoniummade in 2014 when excavationshad finished and the properbasement was fully freed (b)

small wall opening, CO2 concentrations slightly de-creased. Due to some mixing with the normal atmosphereoutside, around 74% CO2 were seen. Due to the extremeCO2 concentrations within the grotto, oxygen concentra-tions were well below ambient; only 6–2% O2 were foundwithin the grotto, the rest being nitrogen (N2).

The situation was totally different for the sanctuary’s floor,the antechamber outside the grotto (Fig. 2a; blue arrow).Surrounded by walls, which prevented the rapid discharge ofthe gas, the floor was continuously flooded with CO2 duringthe whole day. As CO2 is 1.5 times heavier than air, the es-caping CO2 gas forms a gas lake at the floor (Pfanz et al. 2014,Kies et al. 2015). Depending on atmospheric and microclimat-ic conditions, the CO2 gas lake on the floor of the antechambervaries diurnally. It has again to be mentioned that due to theincomplete excavation state of the Plutonium in 2013, thelocation of the floor does not represent the original properfloor level (but see Fig. 2b). Yet, even at this higher level ofthe floor, a small gas lake existed. It has to be stressed that adeadly CO2 gas lake existed only under certain weather con-ditions: no direct sunlight and no strong air movement. Onsunny or windy days, no gas lake was measurable duringdaytime but occurred through the night.

Yet, measurements of the gas lake performed during earlymorning hours clearly proved the presence of CO2. Althoughit was mostly slightly windy during our first campaign, theactual CO2 gas lake had a maximum height of 35 cm abovethe floor of the antechamber with CO2 concentrationsreaching 57% at the very bottom. As expected, CO2 concen-trations dropped with height due to wind dilution. At 10 cmaboveground, CO2 was reduced to 36% and further declinedvia 9.0 and 3.9% CO2 at a height of 20 or 30 cm, respectively.At 40 cm above the ground level, measured CO2 concentra-tions were close to ambient (0.04–0.05% CO2). At the sametime, oxygen values increased from 8.0–9.3% at the very bot-tom via 12.7 and 18.7 to finally reach 19.3–20.7% at 40 cmheight. During all morning hours, observed CO2 concentra-tions at the very bottom of the antechamber of the Plutoniumwere always in the range between 27 and 76%. At the sametime, oxygen values were very low (6.6–12.5%). The reasonfor the broad concentration range of both gases was the fluc-tuating wind and solar irradiation.

In addition, the great number of corpses of insects andbirds corroborated the existence of a deadly CO2 gas lakein front of the grotto. On our first day, two dead birds andmore than 70 dead beetles (Tenebrionidae, Carabaeidae,and Scarabaeidae) were found asphyxiated at the floor(Fig. 3). Locals report on dead mice, cats, weasels, andeven asphyxiated foxes. Most animals are not killed dur-ing sunny days, but during the dark evening and morninghours. For small organisms like insects, there is a dangerto get asphyxiated even during daytime; they were seen tobe asphyxiated even during midday hours. The reason for

this is the persistent thin gaseous CO2 coat that covers thelowest 5 cm of the very bottom throughout the day.

Data from the 2014 campaign

Due to the advanced excavation state of the lower part ofthe sanctuary (Fig. 2b), diurnal gas fluctuations within theantechamber of the Plutonium could be measured contin-uously in 2014 (Table 1). As the floor was flooded withwater from a thermal creek, the gas analyzer was posi-tioned at three different heights above the floor of theantechamber: (i) directly above the water level (15 cm;on stones which emerged from the water table) or (ii)40 cm or (iii) 100 cm aboveground. The data given inFig. 4 show the differences of the diurnal CO2 gas re-gimes at these heights. Varying the height of the measure-ments, the potential threat of the deadly gas to differentorganisms could be simulated. Furthermore, continuousmeasurements exactly proofed at which time of the daythe highest danger was to be expected. As only one CO2

device was available, the data at different heights repre-sent different days (Fig. 4).

When the CO2 analyzer was mounted 100 cm above theground of the antechamber, nearly no excess concentrations ofCO2 were found during a diurnal cycle (Fig. 4).Measured dataranged between 0 and 5% CO2. Nevertheless, it has to bementioned that CO2 concentrations around 5% can lead todizziness in humans if inhaled for longer than 5 min(IVHHN 2013). The picture was clearly different when theCO2 monitor was placed closer to the ground (40 cm above-ground). A dramatic increase of CO2 was now seen during thedark hours. Depending on the prevailing wind and rain con-ditions, CO2 ranged between 2 and 36% (Fig. 4). Yet, in most

Archaeol Anthropol Sci (2019) 11:1359–1371 1363

Fig. 3 Gas victims in front of the grotto. The beetles were killed duringthe high-CO2 phase of the gas lake persisting through the dark hours. As avery thin gas lake (up to 5 cm in height) also exists during daytime,insects also suffocate during sunny hours. Dead beetles were fromseveral families: Tenebrionidae, Carabaeidae, and Scarabaeidae

time of the day, concentrations were below 10% CO2. Thoseconcentrations will asphyxiate and kill humans and oth-er mammals within minutes. Even higher CO2 concen-trations were reached with the monitor placed directlyabove the water phase of the antechamber’s floor(15 cm above the proper floor). During the, night ex-tremely high CO2 were found (Fig. 4). The diurnal CO2

concentrations were neither constant nor were they

stable. But even during daytime, concentrations wererather high (5–20%) but they still increased in the eve-ning hours to reach maximum levels during late eveningand night (Fig. 4). Deadly 68% CO2 were then mea-sured in the atmosphere close to the bottom (sometimeseven 85% CO2; not shown). During morning hours,these concentrations were quickly reduced due to solarirradiation (see also Kies et al. 2015).

Table 1. CO2 and radonconcentrations as measured atselected locations in the vicinity oftwo Sanctuaries. Measurementswere performed in 2014

ID Type Date Information Latitude Longitude Rn(kBq/m3)

CO2

(%)

F-1 Fracture 08/06/2014 Denizli-Hierapolis 37.92559 29.12652 46.880 96.30

F-2 Fracture 08/06/2014 Denizli-Hierapolis 37.92473 29.12670 0.128 0.38

F-3 Sinkhole 09/06/2014 Denizli-Hierapolis 37.92498 29.12681 19.500 22.40

F-4 Sinkhole 09/06/2014 Denizli-Hierapolis 37.92484 29.12651 – 0.10

F-5 Fracture 09/06/2014 Denizli-Hierapolis 37.92490 29.12630 – 0.10

F-6 Fracture 09/06/2014 Denizli-Hierapolis 37.92550 29.12626 – 0.07

F-7 Fracture 09/06/2014 Denizli-Hierapolis 37.92523 29.12686 – 16.40

F-8 Fracture 09/06/2014 Denizli-Hierapolis 37.92468 29.12699 – 0.15

F-9 Sinkhole 09/06/2014 Denizli-Hierapolis 37.92848 29.12469 – 4.50

F-10 Sinkhole 09/06/2014 Denizli-Hierapolis 37.92691 29.12546 87.150 36.00

F-11 Apollo 10/06/2014 Denizli-Hierapolis 37.92681 29.12686 162.070 60.00

F-12 Sinkhole 10/06/2014 Denizli-Hierapolis 37.92547 29.12708 13.950 51.30

F-13 Sinkhole 10/06/2014 Denizli-Hierapolis 37.92589 29.12714 – 65.70

F-14 Sinkhole 10/06/2014 Denizli-Hierapolis 37.92571 29.12712 103.000 81.40

F-15 Sinkhole 12/06/2014 Denizli-Hierapolis 37.92708 29.12684 50.40

Plutonium 08/06/2014 Denizli-Hierapolis 37.92625 29.12704 6.500* 6.40*

08/06/2014 Denizli-Hierapolis 22.210** -

*in air

**in waterFF-9

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Fig. 4 Diurnal courses of the CO2 gas concentrations in front of theantechamber of the grotto within the sanctuary of Pluto. Measurementswere performed for 24 h (from 09.00 p.m. to 09.00 p.m.) at three differentheights aboveground (lowest level 15 cm; medium level 40 cm; andupper level 100 cm). Measurements at the proper ground level were notpossible as the base of the antechamber was flooded with water drainingfrom the hot carbonate creek running downhill through the grotto. The

weather at these days was quite similar. Rather hot and sunny duringdaytime (temperatures up to 38 °C) and strong rain events andthunderstorms in the afternoon or during the night. As only one CO2

device was available, the data at different heights represent differentdays. Average temperature and wind speed are given. Rainfall eventsare marked in red. Rainfall and wind speed values are exaggerated by afactor 10

Radon concentrations in both temples (9 and 55 kBq m−3)can be considered as higher than normal since they are locatedin the same active fracture zone. Radon concentrations mostlyparalleled the concentrations of geogenic CO2 (Table 1). Thisbehavior is understandable as radon is transported from itsgeological source within a matrix of CO2 as the carrier. Itbecomes clear that geogenic gas emissions frequently occurwithin the fracture zones of the graben structure cutting itsway through ancient Hierapolis.

Aside from carbon dioxide, other gases could also be mea-sured within the grotto, but only in very low concentrations.Carbon monoxide was present in minute concentrations (1–3 ppm) and oxygen concentrations were down to 3.2%.Methane and hydrogen sulfide were absent, whereas radonconcentrations were found to be between 50 and55 kBq m−3 in both places. The chemical analysis (Table 2)is also consistent with the in situ measurements. The aircorrected helium isotope ratios of the two samples are 3.61and 3. 78 Rac, respectively. These values are consistent with aprevious work of Ercan et al. 1995 (air corrected R/Rac =3.68) and Gulec et al. 2002. The concentration of helium isabove the atmospheric value and CO2 is the dominant gaspossibly carrying mantle sourced helium. The relationshipbetween 3He/4He and 4He/20Ne ratios shows mixing curvesbetween atmospheric and mantle/crustal components (Fig. 8)assuming that an atmospheric component has 3He/4He =1.39 × 10−6 and 4He/20Ne = 0.318, and a crustal componenthas 0.02 Ra and 4He/20Ne = 1000 (Sano and Marty 1995).The presence of different mantle-type components is also con-sidered: MORB-type mantle with 8 Ra and 4He/20Ne = 1000,sub-vontinental European mantle (SCEM, Dunai and Baur1995) R/Ra = 6.5 and 4He/20Ne = 1000. The mantle contribu-tion was calculated about 75% (Table 2) and the radon contentin the gas samples confirmed deep origin with remarkablyhigher values. It is obvious that this higher helium isotoperatio with an elevated CO2 and radon content strongly hintto the tectonic activity and proves the ascent of mantle vola-tiles to the surface. Moreover, the N2/O2 ratio is well abovethat of atmospheric ratio (~ 2) which is consistent with thehigher helium isotope ratio (Table 2).

The old Plutonium—geogenic gas below the templeof Apollo

Similar findings were obtained when the grotto below thetemple of Apollo was studied. The sanctuary of Apollo issituated 200 m west of the sanctuary of Pluto within the sameseismic fracture zone due to the extension of the fault line andit is long known for its subterranean grotto-like structure(Figs. 1 and 5). This cellar-like chamber below the ApolloSanctuary is marked as BPlutonium^ in touristic guides ofHierapolis (D’Andria 2003). The toxicity of its gas

atmosphere is known and the entrance to the cellar belowthe temple was therefore firmly walled due to safety reasons.

When the Teflon-tubing was inserted 2–3 m into the mouthhole of this grotto, between 60 and 65% CO2 were to bemeasured independent of the exact location of the tubing. Atthe same time, oxygen was around 8–8.5%. Also, here, like inthe neighboring new Plutonium, a deadly geogenic CO2 at-mosphere accumulates within a walled chamber. And again,this gas emission reflects the breath of the deadly underworldin ancient times. Because the originally existing larger open-ing had been sealed due to safety considerations, no enhance-ment of CO2 concentrations could be observed outside thehole.

Geogenic gas in the closer surroundings of the twosanctuaries

Since both sanctuaries are located in the same seismic fracturezone of the Hierapolis fault (De Filippis et al. 2012). Gasanalyses were enlarged to the close vicinity of the temples.Therefore, some measurements were performed within thelinear fracture zone east and west of the two sanctuaries(Fig. 6a). Several CO2-emitting spots were found in sinkholesin the vicinity of the sanctuaries (Fig. 6b). In the south ofPluto’s sanctuary along the graben lineament, gas concentra-tions of up to 84% CO2 were measured in small crevices.Increased CO2 was also found in the seismic graben structureitself but also in sinkholes around the fracture zone. Yet, thehighest CO2 concentrations were found within the grotto ofthe sanctuary of Pluto and at a sinkhole some 25 m east of thesanctuary. Geogenic CO2 decreased in southeast and north-west directions, although concentrations were still deadly untillocations F3 (141 m) and F15 (92 m; Fig. 7a, b). The fracturesand sinkholes in the close vicinity of the two sanctuaries arenot evenly distributed; therefore, the sites for gas measure-ments had to be adapted to the local settings. Since the surveywas focused on the closer vicinity of both temples, more dis-tant natural springs and fractures were excluded.

Discussion

Physical and meteorological behavior of geogenicCO2

Within the two sanctuaries of Apollo and Pluto, geogenic CO2

was found in subterranean chambers (D’Andria 2013). Thesegrottos were situated either below the proper temple (in thecase of the Apollo temple) or below the seating rows for thepilgrims and spectators (in the case of the sanctuary of Pluto).In both cases, more than 60% CO2 accumulated within thewalled grottos as neither sunlight nor wind was able to dilutethis poisonous atmosphere.

Archaeol Anthropol Sci (2019) 11:1359–1371 1365

The gas that permeated through cracks and holes to theoutside of the grotto of the sanctuary of Pluto showed a dif-ferent behavior. Due to the higher density of CO2, the diurnalbuildup of a CO2 gas lake occurred on the very bottom in frontof the grotto.

CO2 concentrations in the antechamber of the grottolocated directly ahead of the sitting rows for spectatorsand pilgrims were highest close to the bottom (Fig. 4).With increasing height, CO2 concentrations decreased.Furthermore, sunlight, temperature, and wind induced adiurnal pattern in the gas lake of the antechamber.

Within the antechamber, the formation of a higher-concentrated CO2 gas lake occurred only during the darkhours of calm nights. Under windy conditions or duringthe sunny hours of a normal day, the gas lake was partial-ly or wholly destroyed due to the strong infrared absorp-tion of CO2 and the concomitant heating of the system(Pfanz et al. 2004, 2014). The heating of the gas lakeleads to a decrease of its density and to its disintegration

and final dissipation within the above-lying air parcels.Similar observations have been made for a larger, naturalCO2 gas lake in Italy (Kies et al. 2015). Furthermore, acloser look to Fig. 4 shows that even within this diurnal,sinus-like BCO2 wave^ of the gas lake, concentrationswere not ideally stable for a longer period. A heavy scat-ter of CO2 concentrations mainly during night hours isseen (Fig. 4). Concentrations varied from 68 to 10% CO2

within minutes. The reason for these sudden CO2 changeswere wind gusts and heavy rain events occurring duringour measurement campaign (see also Bettarini et al.1999; Etiope et al. 2005; Pfanz 2008; Kies et al. 2015).Meteorological data compiled from the region are wellconsistent with the CO2 data. There is an inverse relation-ship between the CO2 gas concentration and air tempera-ture, rainfall, and wind causing a CO2 decrease (Fig. 4).

Table 2 Gas analysis of the Pamukkale spring as determined by the Noble Gas Laboratories of INGV, Palermo

Sample ID He O2 N2 N2/O2 CO CO2 R/Ra He/Ne [He] Corr [Ne] Corr R/Rac Mantle Contribution Rate

% Atm % Rad % Mag

Pamukkale 1 0.0011 3.3 17.9 5.42 0.0005 80.38 3.25 2.290 8.094 3.535 3.61 11.6 13.69 74.68

Pamukkale 2 0.0012 3.43 18.45 5.38 0.0003 80.65 3.33 1.965 6.213 3.162 3.78 13.6 8.52 77.91

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Fig. 5 Radon measuring set in front of the closed chamber of the Apollotemple. The CO2-filled room below the temple has been sealed by walls.Gas measurements were therefore made by inserting the tubing throughthe small window that was left open

Fig. 6 Fracture zone thorough a travertinic rock east of the sanctuaries(a). Gas tubing was inserted into a sinkhole in the vicinity of thesanctuaries (b)

Gate to hell—the historical relevance of the gasfindings

Biological effects of extreme CO2 concentrations

Several old writers report on the death of birds, rams, and bullsin the vicinity of the two Plutonia. The measured data clearlyproof that even nowadays, CO2 concentrations and the con-comitant low oxygen concentrations within the grottos andantechambers of the sanctuaries of Pluto and Apollo are high-ly toxic. So clearly, oxygen-depending life will not exist

within the grotto of Plutonium. Neither mammals nor reptilesor birds can survive. Even insects are killed within minutes(for some exceptions, see Russell et al. 2011). Outside thegrottos, in the antechambers, the situation is different.Depending on the size/height of the respective animal (anddepending on the time of the day and the weather condition),the situation could be either deadly toxic or only slightly dan-gerous. Mammals already react to CO2 concentrations as lowas 3–5%. Even these rather low concentrations may increasecardiac frequency and respiratory rate if incubation time islonger than several minutes (D’Alessandro 2006; Pfanz

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Fig. 7 Location of the differentmeasuring points of CO2 andradon contents within the settingsof the two sanctuaries of Apolloand Pluto. The points wereselected according to themorphology of the site and thedirection of the fissure (a). CO2

and radon concentrations asmeasured at the locations given(b). Data from both campaigns in2013 and 2014. CO2

concentrations are given inpercent; radon is given as kBeq/l.

2008). At CO2, concentrations around 8–10% humans areasphyxiated and a longer incubation at 15–20% inevitablyleads to death (IVHHN 2013).

Historical relevance of the gas findings

Entrance to hell

To the ancient people, these gas-emitting places were extreme-ly frightening and therefore holy and consecrated. Such siteswere often found just accidentally by herdsmen or farmerswatching their cattle behaving strange. Herdsmen were astutein observing nature; even a slight change or variation in thenormal vegetation (Pfanz et al. 2004) could alert them to suchextraordinary places (Pfanz et al. 2014). Often temples andsanctuaries were then erected at these sites. These sacred sitesresembled the entrance to the Underworld, the antechambersof the Hadean. Deadly vapors were escaping from these out-lets (Strabo XIII, 4, 14). The toxic vapors resembled either thedeadly breath of Pluto/Hades, the Greek god of the kingdomof the death or even more plausible the breath of the frighten-ing hellhound Kerberos guarding the entrance to hell.Furthermore, these places were also the sites were Plutoabducted Persephone, the daughter of Demeter (see also asimilar site in Greece, Eleusis; Von Uxkull 1957). The alreadyterrifying impression of Hadean entrances is sometimes cor-roborated by evaporating steam pouring out of holes andcracks on cold days. This is also true for the Plutonium atthe Sanctuary of Hierapolis; a hot carbonate-rich, below-ground creek is the cause of it (Fig. 2b).

It is also known from other authors that sacred Greek andRoman oracles and temples were often located on or close togeologically active zones (De Boer et al. 2001; Etiope et al.2006; Piccardi et al. 2008; Mariolakos et al. 2010).

Understanding the principle of sacrificing bulls, rams,and birds

The toxic atmosphere around these sites lead Greeks andRomans to believe that these places, called mephitic cavesor mephitic locations, were the entrance to the ancientunderworld, the Hades, the ancient gate to hell(Plutonium, Charonium; see Piccardi 2007; Cassius Dio,n.d, Plinius, n.d, Strabo, n.d). As there were celebrationsand festivities, ancient tourists, pilgrims, and spectatorswere attracted. Even at this time, tourism was sophisticat-ed as not only accommodation and food was provided,but also living animals for sacrifice and other attractionswere sold. The spectators of the religious presentationsthrew the freshly purchased birds down onto the ante-chambers floor (into the invisible gas lake) where the tinycreatures quickly died (Zwingmann 2012). Even more im-pressive were the effects of the Hadean atmosphere whenlarger animals were sacrificed. On special events and sa-cred days, bulls and rams were also ushered into the an-techambers of the sanctuary. Young men had carried thesacrificial animal to the sanctuary (see coin found atAcharaka; Von Diest et al. 1913). While the bull wasstanding within the gas lake with its mouth and nostrilsat a height between 60 and 90 cm, the large grown priests

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Fig. 8 Helium and neon isotopicratios given as R/Ra values andHe/Ne relationships, respectively.The theoretical lines representbinary mixing trends ofatmospheric helium with mantle-originated and crustal helium. Theassumed end-member values forHe isotopic ratios mark themixing curves: crust 0.02 Ra andthree different mantle-signatures:8 Ra (MORB), 6.5 Ra (SECM),and 4 Ra for a contaminated/degassed mantle

(Galli) always stood upright within the lake caring thattheir nose and mouth were way above the toxic level ofthe Hadean breath of death. It is reported that they some-times used stones to be larger. The spectators could seethat animals as strong, sturdy, and powerful as bulls diedwithin minutes whereas the priests survived (for anoverview see Pfanz et al. 2014). Many ancient writersdiscuss the survival of the Eunuch priests and state eitherthe godlike powers of the Galli, the application of anti-dotes and/or other precautions to survive the deadly gas(Cassius Dio, Epitome 68.27.2 and 3; for details seeZwingmann 2012). It seems pretty clear that the priestswere fully aware of the gas and also of its physicochem-ical properties. They were aware of the diurnal changes ofthe deadly vapors and they were aware of the varyingheight of the gas lake. For these reasons, it must be as-sumed that religious ceremonies including the sacrifice ofanimals were performed in the evening or early morninghours of calm days. When, on the other hand, the priestwanted to proof their fortitude or invincibility or just theirsimilarity with the gods, they used midday hours for theirperformances. Then they could easily creep deep into thehole of the entrance to hell and by keeping their breath,they could even stay inside for a short while.

Conclusions

The measured 3He/4He isotopic ratios of dissolved gasphase taken from Hierapolis thermal water show a mix-ture of shallow (atmospheric) and deep components eitherof mantle and crustal origin (Fig. 8), with a significantcontribution (about 75%) of mantle-derived helium con-sidering a SCLM-type mantle end member (6.5 R/Ra).This is consistent with the active fault and fractures inthe area.

Furthermore, our measurements strongly corroboratethat at least in the case of Hierapolis, ancient writers likeStrabo or Plinius described mystic phenomena very exactlywithout much exaggeration. More than 2000 years ago,these phenomena could not be explained scientifically butonly by the imagination of supernatural forces fromHadean depths or well-meaning gods.

Measurements of diurnally changing concentrationsand fluxes of the deadly gas emissions corroborate theancient tales of asphyxiated bulls and rams and also thesurvival of the knowledgeable priests. In line with this,many examples for the description of the different oraclesites in proper Greece, Magna Graeca, and Asia Minorcan be found. Nowadays, these phenomena can be ana-lyzed by physicochemical means and explained with theknowledge in geology, chemistry, physics, and biology.

Acknowledgements We want to thank the governorship of Denizli Cityfor their kind help. We are indebted to Omer Faruk Gunay, vice governorof Eskisehir City, and Ismail Soykan, vice governor of Denizli City. Thegreat help of the director of Hierapolis/Pamukkale is greatly acknowl-edged. The authors wish to extend their sincere thanks to the provincialculture and tourism directorate of Ankara and Denizli. Dr. FrancescoItaliano kindly provided data about gas analysis from the grottos.Special thanks to Dr. Christiane Wittmann and Volker Wittmann for theirexcellent work in building a robust, continuously recording CO2 monitor.The authors are also grateful to Selami Yildirim and Mehmet Ergun fromthe Turkish State Meteorological Service. The authors are extremelythankful to Prof. Dr. Francesco D’Andria, who detected and excavatedthe new Plutonium at Hierapolis for his kind help providing a permissionto work on the site, to his intelligent advices and to his permanent interestin the progress of our study.

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