Atoll (from Wikipedia)
An atoll is a type of low, coral island found in tropical oceans and consisting of a coral-algal reef surrounding a central depression. The depression may be part of the emergent island, but more typically is a part of the sea (that is, a lagoon), or very rarely is an enclosed body of fresh, brackish, or highly saline water.
Charles Darwin published an explanation for the creation of coral atolls in the South Pacific (Darwin, 1842) based upon observations made during a five-year voyage aboard the HMS Beagle (1831-1836). His explanation, which is accepted as basically correct, involved considering that several tropical island types—from high volcanic island, through barrier reef island, to atoll—represented a sequence of gradual subsidence of what started as an oceanic volcano. He reasoned that a fringing coral reef surrounding a volcanic island in the tropical sea will grow upwards as the island subsides (sinks), eventually becoming a barrier reef island (as typified by an island such as Bora Bora and others in the Society Islands). The fringing reef becomes a barrier reef for the reason that the outer part of the reef maintains itself near sea level through biotic growth, while the inner part of the reef falls behind, becoming a lagoon because conditions are less favorable for the corals and calcareous algae responsible for most reef growth. In time, subsidence carries the old volcano below the ocean surface, but the barrier reef remains. At this point, the island has become an atoll.
Long Caye is one of only a few atolls in the Caribbean. It also sits at the very edge of the continental shelf. A quarter mile from the island, the ocean bottom suddenly drops 1,000 feet down. The sheer coral wall is amazing and makes for excellent scuba diving although is is rather strange to look below you and see nothing but pitch darkness and to know that it continues down for a very long way.
