Every year, they leave behind a layer of skeleton, which is so tough it almost feels like rock.
That is why some people call coral "living rock".
Scientists say the coral has been dying out because of weather conditions like hurricanes and disease.
Pollution and people doing too much fishing have also led to the decline.
But the scientists didn't find any evidence that global warming had added to the problem.
The good news is they found the damage seems to be slowing down, and some sites even appear to be getting better.