A coral reef looks like a garden of stone, but it is alive. Each piece of coral is a colony of tiny animals called polyps. The polyps share their home with very small plants that live inside them. The plants make food from sunlight and give some of it to the coral. In return, the coral gives the plants a safe place to stay.

When the sea becomes too warm, this partnership breaks down. The coral pushes the plants out and turns white. People call this coral bleaching. A bleached coral is not dead yet, but it is hungry and weak. If the water stays hot for many weeks, the coral dies and the reef slowly falls apart.

This matters to people as well as to fish. A healthy reef feeds millions of families, and it also protects the coast because it breaks the power of big waves before they reach the shore. Villages behind a broken reef lose both their fish and their shield.

Today many groups are trying to help. Divers cut small pieces of healthy coral and hang them on frames in calm water. The pieces grow for about a year, and then divers glue them onto the damaged reef. It is slow work, so scientists say we must also cool the sea by burning less fuel. Growing coral cannot win alone.