Hang Sơn Đoòng, Mountain River Cave, Quang Binh Province, Vietnam

Son Doong cave (in Vietnamese: Hang Sơn Đoòng, meaning Mountain River Cave) is a cave in Phong Nha-Ke Bang National Park, Bo Trach District, Quang Binh Province, Vietnam. The cave is located near the Laos-Vietnam border. It has a large fast-flowing underground river inside.
The 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. However, not until 2009 was it made known to the public when a group of British scientists from the British Cave Research Association, led by Howard and Deb Limbert, conducted a survey in Phong Nha-Ke Bang from April 10-14, 2009. Their progress was stopped by a large calcite wall. According to Limbert, this cave is five times larger than the Phong Nha cave, previously considered the biggest cave in Vietnam. The biggest chamber of Son Doong is over five kilometers in length, 200 meters high and 150 meters wide. With these dimensions, Son Doong overtakes Deer Cave in Malaysia to take the title of the world's largest cave. The cave was named as a World Heritage Site in 2003.