ABSTRACT
The opposite of the hypobaric effect of exercise at high altitude would be exercise in a hyperbaric environment at some place 1000s of meters below sea level. This sort of environment does not exist on the surface of the Earth. The most significant effect of immersion in water is the hydrostatic pressure placed on the body and specifically the gases in the lungs and dissolved in body fluids such as blood. The density of water is such that changes over a few meters are far more dramatic than changes over 1000s of meters in air. Early forms of diving, that still persist both recreationally and in some work environments, relied on breath holding. The limitations of this are obvious as you can only submerge for as long as the time between breaths. The most well known complication of exercise underwater is decompression sickness or the 'bends.' This occurs when a diver ascends too rapidly from a deep dive.
