risks remain. Hazards can be substituted by using a different mode of diving when applicable. Saturation diving is a technique that allows divers to reduce the risk of decompression sickness ("the bends") when they work at great depths for long periods of time, at the cost of substituting other, lower risk, hazards, associated with living in a saturation environment. Freediving Freediving, or breath-hold diving, is the original mode of diving, and was used for centuries in spite of limitations as it was the only option available. It is simple and inexpensive, but severely limited in the time available to do useful work at depth. The risk of drowning is relatively high, as the diver is limited to the oxygen supplied by a single breath, and the risk of hypoxic blackout underwater, followed by drowning, is significant. Hypoxic blackout during freediving is a loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold dive, 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 provoked by hyperventilating just before a dive, or as a consequence of the pressure reduction on ascent, or a combination of these. Victims are often established practitioners of breath-hold diving, are fit, strong swimmers and have not experienced problems before. Divers and swimmers who blackout or grey out underwater during a dive will usually drown unless rescued and resuscitated within a short time. Freediving blackout has a high fatality rate but is generally avoidable. The risk cannot be quantified, but is clearly increased by any level of hyperventilation. Freediving blackout can occur on any dive profile: at constant depth, on an ascent from depth, or at the surface following ascent from depth and may be described by a number of terms depending on the dive profile and depth at which consciousness is lost. Blackout during a shallow dive differs from blackout during ascent from a deep dive in that deep water blackout is precipitated by depressurisation on ascent from depth while shallow water blackout is a consequence of hypocapnia following hyperventilation. Trained freedivers are well aware of this and the competition rules require competitions to be held under strict supervision and with competent first-aiders on standby. However this does not eliminate the risk of blackout. Freedivers are recommended to only dive with a 'buddy' who accompanies them,