Hazards of Diving Deep by Peter Tyson

http://www.pbs.org/wgbh/nova/lostsub/divewave.html

German U-Boat U-869 lies in 230 feet of water, which is fully 100 feet deeper than the recommended maximum limit for recreational diving. Yet this did not stop the team of New Jersey divers seen in the NOVA film "Hitler's Lost Sub" from exploring the wreck over many years. Divers who descend to such extreme depths face a host of hazards. Indeed, the New Jersey team tragically lost three divers in accidents at the U-869 site. In this interactive feature, learn some of the dangers that you would face making a dive to U-869. Please note that this compilation is meant only to give you a rough overview of some of the major hazards divers face. It does not pretend to be comprehensive. Many of the hazards described here are not mutually exclusive (e.g., panic can lead to drowning); others may only pose risks beyond certain depths; still others may apply to the ascent as well as the descent. Finally, many of these hazards are dependent on dive technique. For example, if divers are breathing air, oxygen toxicity is not an issue at depths shallower than 230 feet; if they are diving mixed gas, nitrogen narcosis is not an issue; and so on. For more complete discussions of diving deep, including safety procedures and emergency treatments, we refer you to the references below.
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http://www.pbs.org/wgbh/nova/lostsub/divewave.html