High up in a beautiful karst mountain, directly on the Loei–Nong Bualamphu border, Tham Erawan is an amazing cave shrine reached by a steep but easily climbed stairway of 600 steps. The cave, with a big Buddha statue at its mouth, is very large and a line of lights leads you through the massive chamber and out the other side of the mountain.
From roughly 11am to noon and again around 2pm to 3pm, a hole in the roof of the cave lets down shafts of sunlight, making for a spectacular atmosphere. Don't linger late in the afternoon as the monks are punctual about turning out the lights – we speak from experience.
Buses from Loei (42B, 1¼ hours, every 30 minutes) to Udon Thani will drop you at Ban Pha Wang on Rte 210, 2.5km away from the temple. No túk-túk park here, but it's a fairly pleasant walk.