speleology

Deep in the Vietnamese jungle is a cave so large it has its own weather. Hang Sơn Đoòng — the world's largest cave passage — contains a forest, a river, and a microclimate that forms clouds beneath its ceiling, which soars high enough to fit a 40-storey skyscraper. It was only fully explored in 2009. The largest caves on Earth are among the last great unmapped spaces on the planet, carved by nothing more than slightly acidic water and a great deal of time.