Hang Son Doong is a solutional 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.
Mù Cang Chải is a rural district of Yên Bái Province, in the Northeast region of Vietnam. As of 2003, the district had a population of 42,574. The district covers an area of 1,199 km². The district capital lies at Mù Cang Chải.