Some coral reefs are thousands of feet thick. There are four major types of reefs. The fringing reef grows on rocks along a shore. A sandy lagoon separates the barrier reef from land. An atoll is an irregularly shaped ring of coral with a lagoon in the middle.
Atolls usually form on top of submerged volcanoes.
They are mostly found in deep water in the Indian and Pacific Oceans. The patch reef is located in some shallow, tropical waters. They are small and isolated. There are two types of coral, soft and hard. Soft coral has flexible skeletons that can sway in strong currents.