Carsten Peter/National Geographic Stock

Mountain River Cave (Hang Son Doong): Vietnam

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It wasn’t until 2009 that professional cavers explored the world’s largest cave passage, which stretches for two-and-a-half miles and reaches 300 feet wide and more than 600 feet high. It’s concealed under dense tropical rainforest in Phong Nha-Ke Bang National Park. It may be quiet aboveground, but below, the Rao Thuong River roars through the cave, echoing off its limestone walls.

Why It’s Cool: In several places the cave ceiling collapsed, leaving skylights 100-plus feet in diameter that let sunlight in. Underneath those skylights, jungle environments with their own ecosystems, including fish and insects, evolved.

See More Adventures Anyone Can Do

World's Coolest Caves

Mountain River Cave (Hang Son Doong): Vietnam

It wasn’t until 2009 that professional cavers explored the world’s largest cave passage, which stretches for two-and-a-half miles and reaches 300 feet wide and more than 600 feet high. It’s concealed under dense tropical rainforest in Phong Nha-Ke Bang National Park. It may be quiet aboveground, but below, the Rao Thuong River roars through the cave, echoing off its limestone walls.

Why It’s Cool: In several places the cave ceiling collapsed, leaving skylights 100-plus feet in diameter that let sunlight in. Underneath those skylights, jungle environments with their own ecosystems, including fish and insects, evolved.

See More Adventures Anyone Can Do

Carsten Peter/National Geographic Stock

World's Coolest Caves

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