Thac Bo Cave sits on a limestone peninsula in the middle of the Hoa Binh reservoir, accessible only by boat. The cave itself runs more than 100 metres into the mountain, opening into three large chambers hung with stalactites and stalagmites; the widest point reaches about 20 metres across. It has been recognised as a national heritage site by the Vietnamese Ministry of Culture. Beside it, built into the cliff face, is a small pagoda that was a place of worship for Muong communities long before the dam flooded the valley below.
The approach by boat is half the experience. You cross open water between karst islands, then enter a narrows where the rock walls rise steeply on either side, before arriving at the cave entrance. The cave is cool even in summer. A second cave in the area — Hoa Tien Cave, also classified as national heritage — is accessible on the same boat circuit. Both are best visited in autumn when water levels are high enough for boats to navigate comfortably between the islands.