Sơn Đoòng is a cave large enough to swallow a New York City block, skyscrapers and all, in a single stone chamber over 5 kilometres long, its ceiling rising to 200 metres and its walls standing 150 metres apart. Measured at roughly 38.5 million cubic metres and shown by later dives to be larger still, it is the biggest known cave passage on Earth — one link in a system that runs more than 9 kilometres through the mountains of central Vietnam. A local hunter, Hồ Khanh, found the entrance in 1990 while searching for agarwood, then lost it for years; a British-Vietnamese team led by Howard Limbert finally surveyed it in 2009, and Guinness World Records named it the world's largest cave in 2013. It sits in the core of Phong Nha-Kẻ Bàng National Park, a UNESCO World Heritage Site, and to keep it intact only about a thousand people are let inside each year.
What makes it feel less like a cave than a lost world is light. Twice the ceiling collapsed ages ago, opening skylights nicknamed "Watch Out for Dinosaurs" and "Garden of Edam"; daylight pours through them and has grown true rainforests hundreds of metres underground, loud with birdsong and tangled with vines. Warm air off the river meets cooler air sinking through these holes, so on some days clouds form beneath the stone ceiling. Across the floor runs the Rao Thương, the river that carved the passage and still flows through it; you wade its cold sections, trek jungle inside the mountain and abseil into the first sinkhole, until your headlamp finds a 90-metre wall of glassy calcite — the "Great Wall of Vietnam" that turned back the first expedition. Each January and February, if the morning is clear, shafts of sunlight fall through the skylights and stand in the dark like solid columns.