Many tropical islands are actually the remains of extinct volcanoes that sank beneath the ocean.

The formation of atolls was first accurately described by Charles Darwin in 1842 during his voyage on the HMS Beagle. His theory, known as the subsidence theory, explains that a volcanic island begins as a 'fringing reef' directly attached to the shore. Over millions of years, the cooling oceanic crust becomes denser and causes the extinct volcano to slowly sink in a process called subsidence.As the mountain descends, the coral reef grows upward at a rate of approximately 1 to 10 millimeters per year to remain in the photic zone where sunlight is abundant. This stage creates a 'barrier reef' separated from the sinking land by a lagoon. Eventually, the volcanic peak disappears entirely below the sea level, leaving behind a circular or oval ring of coral known as an atoll.Modern geological studies using deep-sea drilling have confirmed Darwin's hypothesis. For example, researchers at Enewetak Atoll in the Marshall Islands drilled through over 1,400 meters of solid coral before reaching the volcanic basalt base. This proves that the coral has been growing continuously for over 50 million years as the underlying volcano subsided. Today, the Maldives in the Indian Ocean represent the world's largest system of such formations, consisting of 26 natural atolls.