Sơn Đoòng cave (hang Sơn Đoòng, "Mountain River cave" in Vietnamese) is a cave in Phong Nha-Kẻ Bàng National Park, Bố Trạch district, Quảng Bình Province, Vietnam. Currently the biggest known cave in the world, the cave is located near the Laos-Vietnam border (17°27'25.88"N 106°17'15.36"E). It has a large fast-flowing underground river inside.
Sơn Đoòng Cave was found by a local man named Hồ-Khanh in 1991. The local jungle men were afraid of the cave for the whistling sound it makes from the underground river. It was not until 2009 that it became public after a group of British scientists from the British Cave Research Association, led by Howard and Deb Limbert, conducted a survey in Phong Nha-Kẻ Bàng from April 10 through April 14, 2009. Their progress was stopped by a large calcite wall.
According to the Limberts, this cave is five times larger than the Phong Nha cave, previously considered the biggest cave in Vietnam. The biggest chamber of Sơn Đoòng is more than five kilometers long, 200 meters high, and 150 meters wide. With these dimensions, Sơn Đoòng overtakes Deer Cave in Malaysia to take the title of the world's largest cave.