Whale Conjurer, Underwater World, Dominican Republic

History
Although a number of human activities are conducted underwater—such as research, scuba diving for work or recreation, or even underwater warfare with submarines—this very extensive environment on planet Earth is hostile to humans in many ways and therefore little explored. But can be explored by sonar, or more directly via manned or autonomous submersibles. The ocean floors have been surveyed via sonar to at least a coarse resolution; particularly-strategic areas have been mapped in detail, in the name of detecting enemy submarines, or aiding friendly ones, though the resulting maps may still be classified.
An immediate obstacle to human activity under water is the fact that human lungs cannot naturally function in this environment. Unlike the gills of fish, human lungs are adapted to the exchange of gases at atmospheric pressure, not liquids. Aside from simply having insufficient musculature to rapidly move water in and out of the lungs, a more significant problem for all air breathing animals, such as mammals and birds, is that water contains so little dissolved oxygen compared with atmospheric air. Air is around 21% O2; water typically is less than 0.001% dissolved oxygen.
The density of water also causes problems that increase dramatically with depth. The atmospheric pressure at the surface is 14.7 PSI or around 100 kPa. A comparable water pressure occurs at a depth of only 10 m (33 ft.) (9.8 m (32 ft) for sea water.) Thus, at about 10 m below the surface the water exerts twice the pressure (2 atmospheres or 200 kPa) on the body as air at surface level.