Hang Son Doong

You descend eighty metres on ropes into darkness. The headlamp catches the wall, then nothing — the beam vanishing into a void so vast it holds its own cloud layer. On the floor, a jungle grows beneath two collapsed sinkholes where sunlight pours in like stadium floodlights. The scale is wrong; your brain cannot calibrate it. This is the largest cave passage on earth.

Hang Son Doong was found in 1991 by a local logger, Ho Khanh, and first fully explored by a British–Vietnamese caving team in 2009. The main passage runs more than five kilometres, up to two hundred metres high and one hundred and fifty wide — room for a forty-storey building — and holds some of the tallest stalagmites ever recorded, near eighty metres. Two vast dolines let in sun and rain, growing a self-sustaining jungle with its own localised weather. Entry is by licensed expedition only — small teams, days underground, jungle and river passages between the chambers, camp pitched on underground sand beaches beside the subterranean river.