An atoll is a circular ring of coral that encloses a shallow lagoon, it is normally covered by a thin layer of soil. Generally, only tropical plants, like palm trees, can thrive on an atoll. In many cases, atolls are built up on the crater of a volcano that has sunk below the sea. On the side of the atoll that the wind usually blows against, there is a break in the circle so the pool has an opening into the surrounding sea.