Deep-sea diving is an activity in which a person can descend beyond the recreational scuba diving limit of 40 m (130 ft). eyond this depth! safer and more comple" e#uipment must be used. Due to the logistical support and e"penses re#uired! deep-sea diving is normally done for science or profit. Underwater Observation and WorkDeep-sea diving allows divers to perform underwater observation! surveys! or wor$. Diving scientists geologists! biologists! ecologists! physiologists! and archaeologists use deep-sea diving techni#ues and e#uipment both for profit and to further humanity%s $nowledge about the planet &arth. 'ommercial divers perform wrec$ surveys( submarine rescue operations( welding and cutting operations on pipelines! bridges! and platforms( and inspections of piers! platforms! brea$waters! dams! nuclear power plants! and sewer discharge lines. )hey also salvage valuable or polluting cargo from sun$en ships. *n all such diving! one factor must be overcome+ e"posure of the diver to cold water. reathing-gas heaters! body heaters! bell-system insulation and heaters! dry suits with layered underwear! and warm-water suits are all used.The Breathing Mediumreathing gases are delivered using sophisticated systems similar to those worn by astronauts. Delivery may be by means of hoses connected to surface compressors or ban$s of gas bottles! or by means of a surface-supplied bell system! or from bottom-stationed habitats.)he use of compressed air is limited to depths of less than ,- m (./0 ft). )he to"ic effects of o"ygen and the narcotic effects of nitrogen become the depth-limiting factors. )o e"tend the range of divers! various mi"es of other gases with the atmospheric gases have been tried! including hydrogen! helium! argon! and neon. )he difficulty of breathing dense gases at depth! along with dangerous physiological side effects of decompression sic$ness! or bends! when returning to surface atmospheric pressures! become depth-limiting factors as well.0elium-o"ygen mi"es e"tend the range of divers to wor$ing depths of more than --0 m (.!1-/ ft) before the density of the helium becomes a problem. 0ydrogen offers the potential for divers to go beyond the limits set by helium! but accurately controlling the minute percentages of o"ygen re#uired at such depths is both difficult and dangerous. reathing gases may be dumped or else recirculated! filtered! and recharged with o"ygen. 1ome systems are designed to aid diversin inhaling and e"haling the dense gases.Diving Techniques and Equipment1urface-tethered diving re#uires surface support systems and uses lightweight full-face mas$s! fiberglass helmets with nec$ seals! or heavy metal helmets attached to dry-suits. )hese also allowfor diver-to-diver and diver-to-surface communications. 1pecial systems protect divers from contaminated waters. )o limit decompression times! one-atmosphere suits or submarines with manipulator arms are used if these systems can do the 2ob.3or deep dives involving long-duration decompression schedules! so-called saturation diving systems are used. 4n some such 2obs! divers are $ept at dive-depth pressures in surface chambersfor up to -0 days! then are delivered to the dive site by means of a diving bell that limits their e"posure to the water and provides a measure of comfort and protection during the long descent and ascent. )wo divers would rotate duties for up to 5 hours. Divers saturated on helium-o"ygen can anticipate appro"iately .4 hours of decompression for each 33 m (100 ft) of saturation depth.)hat is! 165 m (-00 ft) of saturation depth e#uals - days of decompression.4ne-atmosphere diving systems enable the diver to ascend directly to the surface without concern about the bends or delays for decompression. )he systems provide life support by removing carbon dio"ide and adding o"ygen. )he one-atmosphere 78*97 suit is named for 8im 8arrett! who made the first e"perimental dives with it in the 16.0s. *t and its successors are e"tremely heavy and re#uire surface tethering! thereby limiting their mobility. 1oft ocean bottoms can also limit their usefulness.1ubmersibles with manipulator arms are also used for deep-ocean wor$! as are remote-operated vehicles (:4;s) e#uipped with television cameras! thrusters! and manipulator arms. *n all surface-tethered systems! surface conditions and ocean currents play a ma2or role in whether or not a dive can be made. &"tensive surface support is necessary in all deep-ocean e"ploration techni#ues.ational 4ceanic and ?tmospheric ?dministration! The NOAA Diving Manual! 4th ed. (.000)( 1isman! David! ed.! TheProfessional Diver's Handbook (165-)( 9iller! 8im! and @oblic$! *an! Living and Working in the Sea! .d ed. (166/).See alsobathyscaphe( bathysphere( oceanography.