Discovery In the Deep

Last fall, scientists diving in the waters near Tahiti, an island in the Pacific Ocean, discovered something remarkable—a coral reef “stretching as far as the eye can see,” as one diver said. 

Moreover, it seemed to be in perfect condition—making the nearly 2-mile-long wonder one of the largest known healthy coral reefs on Earth. (Climate change, pollution, and other threats have caused damage to most coral reefs worldwide.) The discovery has thrilled experts who study coral reefs—and hope to save them.

Coral reefs grow along shorelines around the globe. The largest ones are found in warm, tropical waters near the equator (see map, below). 

A coral can be made of up to thousands of tiny jellyfish-like animals called polyps. Each polyp grows a hard shell known as an exoskeleton for protection. A reef begins when a baby coral, called a larva, attaches itself to a hard surface. Additional coral larvae latch on, and bit by bit, over thousands of years, they form a reef.

Coral reefs cover less than 1 percent of the ocean floor. But they provide a home and a food source for about a quarter of all marine animal species and a wide variety of plant life. The most diverse ecosystems on the planet, reefs are often called “the rainforests of the sea” by scientists.

Yet today, coral reefs are “the most endangered group of animals on the planet,” says biologist Nancy Knowlton of the Smithsonian Institution. 

One major threat comes from carbon dioxide (CO₂), a greenhouse gas that is produced when humans burn fossil fuels to power homes, cars, and businesses. Oceans have absorbed much of the excess CO₂ that has built up in the atmosphere in recent decades. The gas turns ocean water acidic, which makes it hard for corals to build shells. This causes coral reefs to break down. 

Increased CO₂ in the atmosphere has also been a major contributor to climate change, which has raised ocean temperatures. Corals rely on tiny plantlike algae that live inside them to produce much of their food. But when seawater becomes too warm, the algae stop making food—or even die—threatening reefs with starvation. One recent study estimates that half of the coral reef area on Earth has vanished since the 1950s. This threatens the vast number of species that depend on reefs—including human beings. Coral reefs are sources of food for about 1 billion people around the world.