Some of the most extensive limestone cave systems in Asia can be found in Laos. One of the most spectacular is the Konglor Cave in central Laos' Khammouane Province. There is a river flowing all the way through this huge 7.5 km long cave and you can ride a small boat right through it. The limestone forest in Khammouane has the highest concentrations of caves in Laos, and in, Tham Pa Fa, 229 Buddha images were discovered in 2004. If you want to explore off-the-beaten path caves, the Pha Laem trek near Tha Khaek is a good option. Vang Vieng, just north of Vientiane is also well-known for karst mountains and caves. In Luang Namtha's Vieng Phoukha, visit easily accessible but little known Phou Prasat or Tham Kao Rao. The most famous caves in Laos are way up in Houaphan Province's Vieng Xay district. These fortress-like caves served as a base for the Lao Revolutionary Forces during the Second Indochina War. Other notable caves are the Tham Pa in Xieng Khouang and Tham Ting in Luang Prabang.