A few hundred meters below the ocean's surface it is dark because light cannot penetrate to those depths. Blue light penetrates water farther than red or violet light, and because more blue light is returned to the surface without being absorbed, the open ocean usually looks blue in color.

Seawater may look blue-green when it contains a large amount of phytoplankton - tiny plant-like organisms that contain chlorophyll. The yellow-brown sea found in coastal regions probably gets its coloring from mud, sand or pollution.

Sound - particularly high-frequency sound - penetrates much farther than light does in the ocean. This property has led to the development of echo-sounders which are used to construct pic­tures of the ocean floor.

Sound can also be used to locate shoals of fish. At a depth of between about 500m and 1,500m a sound channel exists where sound waves can be trapped and along which they travel for large distances. Whales have been known to use the sound channel to commu­nicate with each other when they are thousands of kilometers apart; zoologists believe that their long-distance "songs" may attract others to new hunting or breeding grounds.

The sound channel has also been used by submarines when hiding in the ocean. A submarine floating in or below the sound channel is difficult to detect because most of the noise it makes travels horizontally, and cannot be picked up by ships at the surface.