Great Blue Hole.

Belize’s most famous natural landmark is a giant marine sinkhole that punctures the Lighthouse Reef Atoll with a dramatic circle of deep sapphire. Measuring nearly 1,000 feet across and over 400 feet deep, this geological time capsule is the collapsed remnant of an ancient limestone cave system from the last Ice Age. While Jacques Cousteau put it on the map for advanced scuba divers—who plunge past the shallow, coral-ringed rim to explore massive stalactites suspended in the dark—the Great Blue Hole is just as spectacular from above. Scenic flights reveal the site’s near-perfect geometry against the shallow turquoise of the Belize Barrier Reef, while day-tripping boaters can snorkel the reef's sunlit edges or pair the journey with the white-sand beaches of nearby Half Moon Caye.