Research in Japan has shown that around 70% of the country's largest coral reef is thought to be dead.
The coral have been "bleached" by warm sea temperatures.
Corals are plant-like animals that live on rocks in the sea. They rely on little organisms called algae for food and to give them their vibrant colour.
When the water gets too warm, the algae leaves, making the coral turn white. This is known as coral bleaching.
Last summer, the temperature of the seawater surrounding Japan's Sekiseishoko coral reef was one to two degrees Celsius warmer than it usually is.
Although coral can survive bleaching like this, it makes them a lot more vulnerable to predators and they can catch diseases. If the water doesn't return to normal temperature quickly, they all starve and die.
Some of Japan's coral areas are now recovering because temperatures have lowered.
However, Japan's Environment Ministry says it's unsure whether this reef will recover.