Scuba diving
Scuba tank, (one or more) containing air.  The air is ordinary compressed air for most dives, but for longer dives a mixture of air with more oxygen, called nitrox can be used to avoid decompression sickness, also called the bends (a painful or deadly problem from going to the surface too fast). The tank is attached to the diver by a harness, which is often part of the BCD. For deeper dives a breathing gas containing helium can be used to prevent nitrogen narcosis, which can make the diver behave as if drunk, and is dangerous.