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.

• How acidic rainwater dissolved permo-carboniferous limestone to create a void large enough for a skyscraper
• Why a roaring underground river and howling wind deterred its discoverer for 18 years
• The Great Wall of Vietnam, world’s tallest stalagmites, and baseball-sized cave pearls beyond it
• How dolines create skylights, underground jungles, and the cave’s own internal weather system
• The cancelled cable car proposal and the debate over loving a natural wonder to death