Hồ Khanh was out after agarwood on 10 December 1990 when he found a cave mouth pushing out cold wind and the sound of a river. The wind and the noise put him off, he lost the spot, and it took him until 2008 to find it again. He led the British Cave Research Association there in 2009 and they measured the largest cave passage in the world by volume, with a cross-section reckoned at twice that of the next biggest, in Deer Cave in Malaysia. The main passage runs over 5 km, 200 metres high and 150 wide, in a system about 9 km long and 150 deep. Two stretches of ceiling have collapsed into dolines, and daylight comes through them, so there is jungle growing underground with trees in it, and stalagmites up to 80 metres, among the tallest known. Tours started in August 2013 at 3,000 dollars a head, permits run January to August only, and the monsoon closes it after that. Quang Tri province, near the Laos border.
Quang Tri, Vietnam
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