An atoll is a ring-shaped coral reef, island, or series of islets that encircle a lagoon partially or completely. Atolls are formed from the growth of coral reefs around the rim of a submerged volcanic island. Over time, as the island erodes and sinks, the coral continues to grow, creating a lagoon in the center. Atolls are typically found in warm, tropical ocean waters, particularly in the Pacific Ocean and Indian Ocean. They provide unique ecosystems that support diverse marine life, including fish, birds, and other wildlife. Atolls are also important for coastal protection and can be vulnerable to climate change and rising sea levels.