Did you know the Maldives is home to the largest coral atoll system on Earth? The Indian Ocean nation has 26 atolls, including Huvadhu Atoll, the biggest in the world by island count, with 255 islands spread across more than 1,200 square miles (3,152 sq km).
Atolls form when underwater volcanoes, called seamounts, sink back into the ocean. As the volcanic island subsides, coral reefs grow upward, creating a ring-shaped reef with a lagoon in the center.
While the Maldives may be the most famous, atolls are scattered across the Indian and Pacific oceans, with a handful in the Caribbean but none in the Atlantic. From above, they resemble sapphires floating on turquoise waters. Beneath the surface, they reveal thriving ecosystems of tropical marine life in some of the cleanest waters on the planet.
Yet these fragile paradises face mounting threats. Rising sea levels could make many uninhabitable within decades, while plastic waste, like the growing Great Pacific Garbage Patch, endangers their reefs.
But back to atolls and their beauty. Click through this gallery for an engaging journey to some of the most captivating atolls on the planet.