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Underwater

Although a number of human activities are conducted underwater—such as 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.

The fact that human lungs cannot naturally function underwater is but one reason we cannot easily visit the underwater realm. Unlike the gills of fish, human lungs are adapted to the exchange of gases at atmospheric pressure, not liquids. Aside from our 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 weight of a column of air 1 square inch in cross-section and extending from the planet surface all the way to the top of the atmosphere is around 14.7 lbs (the weight of air on 1 square centimeter of the surface is 1 kg giving a pressure of 1 Bar). A comparable weight (and thus pressure) of water is only 10 m or 34 ft (33 for sea water) high. Thus, the water at 33 or 34 feet deep underwater exerts twice the pressure (2 atmospheres or 2 bar) on the body as air at the water surface (1 atmosphere or 1 bar).

For solid objects like our bones and muscles, this added pressure is not much of a problem; but it is a problem for any air-filled spaces like the mouth, ears and lungs. This is because the air in those spaces is compressed under pressure and so does not provide those spaces with support from the higher outside pressure. Even at a depth of 8 feet (2.5 m) underwater, an inability to equalize air pressure in the middle ear with outside water pressure can cause pain, and the tympanic membrane can rupture at depths under 10 ft (3 m). The rate of pressure change is greatest at the surface of the water, for example the pressure increase between the surface and 10m (33ft) is 100% (1 bar to 2 bar) but the pressure increase from 30m (100ft) to 40m (130ft) is only 25% (4 bar to 5 bar).

Any object immersed in water is provided with a buoyant force that counters the force of gravity, appearing to make the object less heavy. If the overall density of the object exceeds the density of water, the object sinks. If the overall density is less than the density of water, the object rises until it floats on the surface.

With increasing depth underwater, sunlight is absorbed, and the amount of visible light diminishes. Because absorption is greater for long wavelengths (red end of the visible spectrum) than for short wavelengths (blue end of the visible spectrum), the color spectrum is rapidly altered with increasing depth. White objects at the surface appear bluish underwater, and red objects appear dark, even black. Although light penetration will be less if water is turbid, in the very clear water of the open ocean less than 25% of the surface light reaches a depth of 10 m (33 feet). At 100 m (330 ft) the light present from the sun is about 0.5% of that at the surface.

The euphotic depth is the depth at which light intensity falls to 1% of the value at the surface. This depth is dependent upon water clarity, being only a few meters underwater in a turbid estuary, but may reach 200 meters in the open ocean. At the euphotic depth, plants (such as phytoplankton) have no net energy gain from photosynthesis and thus cannot grow.

At depths greater than a few hundred meters, the sun has little effect on water temperature, because the sun's energy has been absorbed by water at the surface. In the great depths of the ocean the water temperature is very cold. In fact, 75% of the water in the world ocean (the great depths) has a temperature between 0 and 2°C.

Water conducts heat around twenty five times more efficiently than air. Hypothermia, a potentially fatal condition, occurs when the body's core temperature falls below 35 °C. Insulating the body's warmth from water is the main purpose of diving suits and exposure suits when used in water temperatures below 25 °C.

Sound is transmitted about four times faster in water (1435 m/s) than it is in air (330 m/s). The human brain can determine the direction of sound in air by detecting small differences in the time it takes for sound waves in air to reach each of the two ears. For these reasons divers find it difficult to determine the direction of sound underwater.

• See also: Timeline of underwater technology

• Dueker, C. W. 1970, Medical aspects of sport diving. A.S. Barnes and Co., New York. 232 pp.
• The Briny Deep – Oceanography notes at San Jose State University.

• Underwater – a popular trance anthem by Delerium.
• Underwater (band) – a band formed in the late 1990s with Jeremy Wilkins and Melissa Mileski.
• Underwater – a graphic novel by Chester Brown.