The largest cave passage on Earth. Son Doong in Phong Nha-Ke Bang, Vietnam, is big enough to hold a city block, with its own jungle growing under collapsed skylights, its own weather and clouds, and a river running through it; it was only fully surveyed in 2009. Access is by a single licensed operator, a multi-day expedition, strictly limited to protect it. Neighbouring Paradise Cave and Phong Nha are open to ordinary visitors and are spectacular in their own right.
Vietnam
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