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Penetration diving or overhead diving is a type of diving where the diver enters a space from which there is no direct, purely vertical ascent to the safety of breathable air of the atmosphere at the surface. Several types of dives meet this condition: cave diving, wreck diving, ice diving and diving other man-made, underwater structures or enclosures. The fact that the diver cannot immediately ascend to breathe from the atmosphere at the surface makes this type of diving much more risky than open-water diving. It is essential that divers doing penetration dives on scuba are appropriately equipped and trained in technical diving techniques such as: * using redundant breathing gas sources * using distance lines to guide the diver to the exit When the dive depth and time are extended, the use of breathing gases other than air can reduce the risks. Nitrox is used for relatively shallow diving and accelerated decompression. Breathing gases containing helium such as trimix are used for deeper dives to reduce the enervating effects of nitrogen narcosis. ==See also== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Penetration diving」の詳細全文を読む スポンサード リンク
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