Where is the longest known underground river?
The longest known underground river was recently discovered in the remote mountains of Vietnam. It’s the Son Trach River, which runs underground for an astonishing seven miles (11 km) through a gigantic cave called Hang Khe Rhy (Grass Stream Cave). The expedition that discovered and partially mapped the Son Trach River spent twelve days underground, plunging through ice-cold rushing water and painstakingly mapping every curve and grotto they found. Son Trach River and Hang Khe Rhy cave form part of the Phong Nha massif, a geological unit that contains some of the world’s most spectacular karst limestone formations. The caves, passages, and above-ground formations in karst areas are formed by the action of water, which dissolves parts of the rock while leaving other parts behind.
