In the rocks, just opposite Gara Lakatnik, about 30 m above the river, is the opening of one of the most famous caves in Bulgaria – Temnata Dupka. It can be reached by a secured by rail, easily accessible trail. The total length of the cave varies, in different opinions, between 5,000 and 9,000 m.
The cave system is three-storey and consists of two highly branched galleries connected to each other. Special sights of the cave are the cross sections in the course of the galleries. In the cave there are many lakes as well as an underground river.
In Temnata Dupka there are almost 20% of all troglobites found in Bulgaria. Of these, the most prominent is Protelsonia lakatnikensis. In 1962 the cave was declared a natural landmark.
