Self-tutoring about geography: the tutor mentions the term atoll.
An atoll, in its classic sense, is a ring-shaped island or group of islands surrounding a lagoon. The idea is that a volcanic outlet lies at or just below the ocean surface. Coral grows on the volcano’s rim, emphasizing its circular outline just above the ocean surface. The volcano’s crater remains full of water, resulting in the lagoon.
Source:Oracle Tutoring by Jack and Diane, Campbell River, BC.