Sơn Đoòng cave is a cave in Phong Nha-Kẻ Bàng National Park, Bố Trạch District, Quảng Bình Province, Vietnam. As of 2009 it is the biggest known cave in the world, and is located near theLaos–Vietnam border (17°27'25.88"N 106°17'15.36"E). Inside is a large, fast-flowingunderground river.
Sơn Đoòng Cave was found by a local man named Hồ-Khanh in 1991. The local men were afraid of the cave for the whistling sound caused by an underground river. It was not until 2009 that it became public knowledge, after a group of scientists from the British Cave Research Association, led by Howard and Deb Limbert, conducted a survey in Phong Nha-Kẻ Bàng from 10 to 14 April, 2009. Their progress was stopped by a large calcite wall.