Diving with the deep sea
First, without oxygen cylinders to help, completely rely on the first hold breath, diving into the deep sea. Under normal circumstances, even carrying oxygen cylinders, because of the huge pressure on the seabed, and security can not be a stable guarantee, thus more visible, unarmed diving on the capacity requirements of how high;
Second, complete unarmed. The sea diving to a certain extent, will have to guide and help the traction rope or other tools, and is completely free diving, need to complete bare-handed. Not only does it need diving, it also needs to surface itself;
Third, lost contact with shore for long periods of time. When the unarmed diver reaches the bottom of the sea, he loses contact with his partner on the ship or shore, and he is not able to know the details of what he is under the water. Usually is over a certain time the diver has not evolved, that is in danger, so as to implement the rescue, but in this case the rescue success rate will not be too big.