A Two-Story Diorama This calm, quiet scene conceals a riot of activity taking place just beneath the surface. To see what's happening in the coral reef below the water level, visit the lower level of this unique two-story diorama.

A chunk of weathered limestone emerges above the waterline at low tide, providing a hint of the reef below. Snails and leathery, oval-shaped chitons (Acanthopleura granulata) scrape algae off the rock with filelike teeth, slowly deepening depressions worn by water and sand, while a yellow-crowned night-heron (Nyctanassa violacea) feeds on crabs.

On a nearby island, enough sand has collected for salt-tolerant plants to take root, causing more sand to build up. Drifting coconuts have populated the island with palms, and greater flamingos (Phoenicopterus ruber) fly in to feed in the shallow lagoon. In addition to distributing seeds, ocean currents also spread the larvae of many animals, including corals, helping to build and populate new reefs.

Controlling the Climate
Ocean currents transport heat from tropical areas around the globe. These immense currents moderate the temperature of colder regions and shape the global climate.

Warm water from the sun-drenched Caribbean is swept northeast by the Gulf Stream, bringing warmth to the Atlantic coast of North America and then on to Europe. This warm current is one reason European cities such as Lisbon, Portugal, are warmer than U.S. cities at similar latitudes, including New York. Stray tropical fishes swept north by these currents can sometimes be found off Long Island, New York, in the autumn, though these waifs will not survive the winter cold.

One major concern raised by global warming is that water flowing south from melting arctic ice could slow or stop the immense currents known as the "ocean conveyor belt" that bring heat to Europe and North America. The loss of this heat from the tropics could potentially plunge these regions into another ice age.

The Gulf Stream This satellite image shows warm water (red) being swept northeast by the Gulf Stream. Swirling eddies that split off from the Gulf Stream carry warm water as far north as Newfoundland, Canada. © NASA/Photri

Guarding the Shore
Coral reefs protect and shelter coastal habitats from powerful waves that sweep in from the ocean. Waves contain tremendous energy and can do immense damage to an unprotected coastline.

In this scene, the water at far left appears dark blue, indicating that it is much deeper than the shallow lagoon at right. Choppy white wave crests reveal where underwater reefs absorb the energy of big ocean swells. Wave breaks like these create calm, protected lagoons between coral reefs and the shore.

By slowing waves that surge in from the open sea, coral reefs protect the shallow habitats behind them, such as mangroves and seagrass beds, where abundant animals—including many vulnerable juveniles—find food and shelter. Coral reefs also protect human communities in coastal areas from devastating storms.

CONSERVATION: Rain from the Sea
In the background of this diorama, rain clouds release their moisture over Andros Island, the largest landmass in The Bahamas. These clouds carry freshwater that evaporated over the oceans; the salt contained in seawater is left behind during evaporation. When ocean winds carrying this evaporated moisture reach a continent or other large landmass, the humid air is often directed upward by mountain ranges, or by warm air rising from sun-baked land. The humid air cools as it rises, causing water vapor to condense into droplets and form rain clouds.

Most of the freshwater on Earth is the result of evaporation from the sea and precipitation over land. The oceans are the main reservoir of water on Earth. About 97 percent of all water on Earth is salty seawater, and most of the remaining water is locked in the polar ice caps. Yet people are consuming the world's freshwater faster than it is being replaced, creating an environmental crisis in many parts of the world.