content: 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.[1][2][3] 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.  The graphs partway down the page give a good idea of what is happening. Basically for some reason the amount of CO2 in the swimmer's bloodstream started out low when he went underwater. Reaching a high level of CO2 in your bloodstream causes that urge to breathe. By starting the dive with a low CO2 level, the body did not produce enough to trigger that urge and so the swimmer did not realize that he was running low on oxygen. Eventually he depletes enough oxygen to black out without ever having realized he needed to come up for air.