A coral reef was a large patch of coral that acted as a marine ecosystem.
The 2047 Hermosa Quake sank a large part of Los Angeles 200 feet underwater, which led to the creation of one of Earth's largest coral reefs, home to thousands of marine lifeforms. (VOY: "Future's End")
Bracas V had coral reefs. In 2367, Geordi La Forge recalled skindiving there. (TNG: "The Loss")