The underwater world is not a hospitable one for humans. Whereas on land, oxygen is freely available in the air we breathe, the oxygen dissolved in water is not available to humans. Breathing in water will produce drowning. As a result, we must hold our breath when underwater, or use artificial means of oxygen delivery like a SCUBA tank. Humans can hold their breath underwater for only a short time. A highly trained person might be capable of holding their breath for two minutes. Other mammals, such as seals and whales, are capable of holding their breath for much longer, up to about an hour. One reason for these mammals' diving prowess may be the ability of the lungs to collapse under the high pressures of the underwater depths and then to reinflate easily upon return to the surface. Also, the muscles of seals can hold...