Narcosis while diving (also known as nitrogen narcosis, inert gas narcosis, raptures of the deep, Martini effect), is a reversible alteration in consciousness that occurs while diving at depth. The Greek word ναρκωσις (narcosis) is derived from narke, "temporary decline or loss of senses and movement, numbness", a term used by Homer and Hippocrates. Narcosis produces a state similar to alcohol intoxication or nitrous oxide inhalation, and can occur during shallow dives, but usually does not become noticeable until greater depths, beyond 30 meters (100 ft).
Apart from helium and probably neon, all gases that can be breathed have a narcotic effect. This effect is consistently greater for gases with a higher lipid solubility and there is good evidence that the two properties are mechanistically related. As depth increases, the mental impairment may become hazardous. Although divers can learn to cope with some of these effects, it is not possible to develop a tolerance. While narcosis affects all divers, predicting the depth at which narcosis will affect a diver is difficult, as susceptibility varies widely from dive to dive, and between individuals.
When narcosis appears, it may be completely reversed in a few minutes by ascending to a shallower depth with no long-term effects. For this reason, narcosis while diving in open water rarely develops into a serious problem as long as the divers are aware of its symptoms, and may ascend to manage it. Diving beyond 40 m (130 ft) is generally considered outside the scope of recreational diving: as narcosis and oxygen toxicity become critical factors, specialist training is required in the use of various gas mixtures such as trimix or heliox.