Going and staying beneath the surface of water, ranging from breath-hold free diving to scuba diving with air tanks and deep-sea work using submersibles.
Submerging your face in cold water triggers the "diving reflex." Heart rate drops 10-25% and blood flow shifts to protect the brain and heart. This ancient mammalian reflex is especially strong in babies under six months.
Diving deep into the ocean causes symptoms like alcohol intoxication — the "martini effect." Every 15 meters of depth supposedly equals one martini. Rising water pressure dissolves more nitrogen into the blood, producing a narcotic effect on the nervous system.