Imagine flying a Boeing 747 through a subterranean cavern without the wings scraping the sides, or dropping an entire New York City block of 60-story skyscrapers into a hole in the earth with room to spare. That is the scale of Hang Son Doong in Vietnam, the largest natural cave on the planet, a colossal lost world that stayed hidden from the modern world until just over 30 years ago.

Carved over millions of years by chemical erosion and a subterranean river, the cave was found by a local man hunting agarwood in 1990, lost again, and not fully mapped until British researchers led expeditions starting in 2009. Inside lie record-breaking formations, collapsed skylights that grow entire jungles underground, and a fierce modern battle between conservation and mass tourism.

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