A forest inside
Under two roof openings.
Trantuanviet — CC BY-SA 4.0 · Doug Knuth from Woodstock, IL — CC BY-SA 2.0 · Linhcandng (thảo luận) — CC BY-SA 3.0 · Dave Bunnell — CC BY-SA 4.0
Son Doong is the largest known natural cave in the world and the largest cave passage by volume: its main passage runs more than five kilometres, up to 200 metres high and 150 metres wide, room enough for a Boeing 747 to fly through. A local man, Ho Khanh, found its entrance in 1990 while searching for agarwood, lost the spot, found it again in 2008, and in 2009 led a British Cave Research Association expedition to it. Two openings where the roof has collapsed let sunlight in, and a forest has grown beneath them inside the cave. It holds some of the tallest stalagmites known, up to 80 metres, and the Great Wall of Vietnam, a 60-metre wall of flowstone near the end of the passage. The only way in is the single licensed expedition, run by Oxalis from January to August; the cave is closed from September to December. The trip lasts six days and five nights, with one night camping in the forest and two inside the cave, and passes Hang En, the world's third-largest cave, on the first day. Groups are capped at ten, ages 18 to 70, and you need excellent fitness and a previous overnight trek with camping; places sell out many months ahead.
Under two roof openings.
A 60-metre flowstone wall.
The world's third-largest cave.
Near the Laos border, Phong Nha-Ke Bang National Park
No. Entry is limited to the licensed six-day expedition run by Oxalis, at USD 3,000, from January to August only.
Paradise Cave and Phong Nha Cave can be seen in a day, and Oxalis also runs shorter overnight treks to other caves in the park.