World's Coolest Caves
Carsten Peter/National Geographic Stock
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.