Deep in the countryside, 61km southwest of Nong Khai, the wooded slopes around BAN PHU are dotted with strangely eroded sandstone formations which have long exerted a mystical hold over local people. Many of the outcrops were converted into small temples from the seventh century onwards, and were probably caused by under-sea erosion some fifteen million years ago. The rock formations all fall under the Phu Phra Bat Historical Park (daylight hours; free), and a well-signposted network of paths connects 25 of them. Among the most interesting are Tham Wua and Tham Khon, two natural shelters whose paintings of oxen and human figures suggest that the area was first settled at least 6000 years ago. The spectacular Hor Nang Ussa, a mushroom formed by a flat slab capping a five-metre-high rock pillar, is thought to be a Dvaravati-era shrine (seventh to tenth century).

The rocks are hard to reach by public transport. From Nong Khai, take an Udon Thani-bound bus for about 35km to Ban Ngoi, and then change onto one of the half-hourly buses from Udon to Ban Phu; from Ban Phu, it's another 13km west to the historical park, which makes a difficult hitch or a hairy ride on a motorbike taxi. The total journey takes a couple of hours.