But when the tide is low, you can see a coral reef on Nikumaroro Island.
It’s 2,600 kilometers long, and it’s the world’s largest coral reef – and coral is a living thing.
Scientists study the ecology of Madagascar to learn how the warmer ocean affects the coral reefs.
This is bad for coral, and so it’s bad for the animals on coral reefs, too.
He found it a literal sandbank, surrounded by a coral reef mostly submerged.