Phou Pha Thi, Mountain summit in Houaphan Province, Laos

Phou Pha Thi is an isolated mountain summit in Houaphan Province standing at approximately 1,786 meters, distinguished by steep limestone cliffs that rise on all four sides of this karst formation. The rocky terrain and dense vegetation surrounding it create a dramatic natural landscape.

The French colonial army used the site until North Vietnamese forces seized it in 1962; the Hmong "Secret Army" retook the area and a CIA airstrip was built by 1966. In 1966 the US Air Force installed a TACAN radio station and an AN/MSQ-77 radar on the summit to guide B-52 raids on North Vietnam, designated Lima Site 85. On 10 March 1968 North Vietnamese commandos and Pathet Lao units overran the lightly defended post, and the US Air Force bombed the mountain for a week to erase traces of the installation. Washington only admitted LS85 had existed in 1996.

The mountain is called Phou Pha Thi, from the Lao "pha" (cliff) and "thi", and the Hmong population of Houaphan Province regards it as sacred, the seat of powerful phi or spirits. The name and the belief belong to the communities living around the massif, near the Vietnamese border.

All four sides of the karst are sheer cliffs 50 to 200 metres high, with sections between 75° and 90°, so the summit is not a walking destination. The mountain lies in Houaphan Province near the Vietnamese border, about 160 km south of Dien Bien Phu and 240 km west of Hanoi. Access is by road from Samneua, roughly 40 km away; the surrounding ridges reach 1,392 to 1,586 metres.

This summit stands completely isolated, positioned roughly 3 to 4 kilometers away from the nearest neighboring ridges. This isolation makes it a striking landmark that stands out visibly from far distances.