What is the geographical term for a ring-shaped coral reef?

Absolutely beautiful, and the water temperature is warmer. The amount of marine life found is amazing. Best place to go snorkeling.

Wen, The water is warmer because it is a volcano underneath! The warm water attracts the Flora and Aquatic life, and the rings help provide protection from Deep water predators!

I got this one wrong. But, since it's a volcano I don't think I'd want to stay there long