Hang Sơn Đoòng
Sơn Đoòng Cave (Vietnamese: hang Sơn Đoòng, IPA: [haːŋ ʂəːŋ ɗɔŋ͡m]), in Phong Nha – Kẻ Bàng National Park, Quảng Trị Province, Vietnam, is the world's largest natural cave. Located near the Laos–Vietnam border, Hang Sơn Đoòng has an internal, fast-flowing subterranean river and the largest cross-section of any cave, worldwide, believed to be twice that of the next-largest passage.