Son Doong Cave, Vietnam

World's largest known cave!

Sơn Đoòng Cave (Vietnamese: Hang Sơn Đoòng ([haːŋ˧ ʂəːn˧ ɗɔ̤ŋ˨˩]); 'cave of the mountain river'[1] or 'mountain cave of Đoòng [village]' in Vietnamese) is a solutional cave in Phong Nha-Kẻ Bàng National Park, Bố Trạch District, Quảng Bình Province, Vietnam. As of 2009 it has the largest known cave passage cross-section in the world,[2][3] and is located near the Laos–Vietnam border. Inside is a large, fast-flowing subterranean river. It was formed in Carboniferous/Permian limestone[4] and is believed to be between 2 and 5 million years old[5].