Shallow water blackout

Shallow water blackout is a loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold dive in water typically shallower than five metres (16 feet), when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have caused it. It can be caused by taking several very deep breaths, or hyperventilating, just before a dive. Victims are often established practitioners of breath-hold diving, are fit, strong swimmers, and have not experienced problems before.