An atoll is a ring-shaped coral reef that encircles a lagoon, typically formed from the remains of a volcanic island. Over time, as the island erodes and sinks, the coral continues to grow upward, creating a barrier around the shallow water. Atolls are often found in tropical and subtropical regions, providing unique ecosystems for various marine life. These structures can vary in size and shape, with some atolls being just a few hundred meters across, while others can span several kilometers. Atolls are important for biodiversity, serving as habitats for fish, birds, and other wildlife, and they also play a role in protecting coastlines from erosion and storm surges.