Answer:
Explanation: An atoll is formed around the mouth of an underwater volcano (which has eroded or subsided beneath the water). A lagoon of water is formed over the caldera of the volcano, while the higher rim of such volcano continues to be underwater. This enables corals to grow and form the coral reefs. Coral reefs have flourishing ecosystems, whereas atolls are commonly known as “underwater deserts” due to their lack of life.