Sơn Đoòng is a cave large enough to swallow a New York City block, skyscrapers and all. Its main chamber runs more than 5 kilometres, with the ceiling 200 metres up and the walls 150 metres apart. At roughly 38.5 million cubic metres, and larger still by later surveys, it is the biggest known cave passage on Earth — one link in a system running over 9 kilometres through the mountains of central Vietnam. Two stretches of that ceiling collapsed long ago, and the holes they left let daylight down onto the floor. Real rainforest grows there, hundreds of metres underground, loud with birdsong and tangled with vines. A river, the Rao Thương, runs the length of the passage; it cut the cave and is still cutting it.
The two skylights are nicknamed Watch Out for Dinosaurs and Garden of Edam. On clear mornings in January and February the sunlight coming through them stands in the dark like solid columns. Reaching them takes days. You rope 80 metres down the entrance shaft, trek jungle inside the mountain, wade cold sections of the river and sleep two nights on the cave floor. Deep in, the route ends at a 90-metre wall of glassy calcite that stopped the first expedition and got named the Great Wall of Vietnam. Hồ Khanh, a local hunter, found the entrance in 1990 while looking for agarwood, then could not find it again for years. A British-Vietnamese team led by Howard Limbert surveyed it in 2009, and Guinness listed it as the world's largest cave in 2013.