"Every one must be struck with astonishment, when he first beholds one of these vast rings of coral-rock, often many leagues in diameter...which day and night is lashed by the breakers of an ocean never at rest."—Charles Darwin

Raccoon Butterflyfish. Rod Mickens / AMNH

Corals may look as solid as rock - but they are actually alive and extremely fragile. And they are under severe threat all around the globe. Pollution can lead to fatal diseases for the reef and its many inhabitants, and soil runoff from deforested areas can choke coral with sediment. Whole reefs are sometimes intentionally poisoned or blasted with dynamite so reef animals can be more easily collected for food or the aquarium trade. Global warming may pose one of the most direct dangers to corals, as the symbiotic algae that live inside coral polyps and nourish them are vulnerable to temperature change. Without these algae - which give corals their color - the coral "bleaches," turning ghostly white and eventually starving to death.

Alarming episodes of mass coral bleaching have occurred in many reefs around the globe. Slight changes in water temperature—caused in part by human-induced global warming—can turn coral white, due to the loss of the dinoflagellate algae that live inside the coral. Without these algae most coral cannot survive for long.

The Tubbataha Reef National Marine Park in the Philippines was created to combat the destruction caused by the use of explosives and poison to gather coral reef animals. After more than half the coral cover in Tubbataha was lost in the 1980s, a complete ban on fishing was instituted in 1997 to protect the reef and help it recover. Such bans can also help local economies by increasing tourism and enriching nearby fisheries.