Which mechanism is primarily responsible for unconsciousness during deep dives?

The mechanism primarily responsible for unconsciousness during deep dives is nitrogen narcosis. This phenomenon occurs due to the increased partial pressure of nitrogen at greater depths, leading to a narcotic effect on the central nervous system. As divers descend, the nitrogen that is inhaled in the compressed air becomes more soluble in the body’s tissues and exerts a narcotic effect, which can impair judgment, coordination, and consciousness. Symptoms may resemble those of intoxication, causing divers to experience confusion, dizziness, and potentially a loss of consciousness if the depth and duration of exposure are significant enough.

The other options, while related to diving, do not directly cause unconsciousness in the same primary manner as nitrogen narcosis. Barometric pressure changes can affect a diver's physiology but do not typically result in immediate unconsciousness; rather, they relate to other potential risks like barotrauma. Hypothermia results from prolonged exposure to cold water, which can lead to unconsciousness over time, but it is not an immediate effect related to depth alone. Decompression sickness, caused by nitrogen coming out of solution during ascent, can lead to serious complications, including unconsciousness, but it is a consequence of rapid ascent rather than a direct effect of the conditions at depth.