The sea floor is home to many unique communities of plants and animals. Most of these marine ecosystems are near the water surface, such as the Great Barrier Reef, a 2,000-km-long coral formation off the northeastern coast of Australia.

Coral reefs, like nearly all complex living communities, depend on solar energy for growth (see How Plants Work?). The sun's energy, however, penetrates at only about 300 m below the surface of the water. The relatively shallow penetration of solar energy and the sinking of cold, subpolar water combine to make most of the deep ocean floor a frigid environment with few life forms.

The sea or ocean looks blue because of sunlight reflected off tiny particles in the water. The sea can also appear green (in the tropics for example) when blue mixes with yellow pigments in microscopic floating plants.

The Black Sea appears black because it has little oxygen and lots of hydrogen sulphide. The Red Sea was named after the seasonal blooms of red algae that tint the surface water.