Some of the most extensive limestone cave systems in Asia can be found in Laos. The limestone forest in Khammouane has the highest concentrations of caves in Laos. Vang Vieng, just north of Vientiane is also well-known for karst mountains and caves. But the most famous caves in Laos are way up in Houaphan Province's Vieng Xay district. These caves served as a base for the Lao Revolutionary Forces during the Second Indochina War and were transformed into shelters and bunkers.