Temnata dupka, Gara Lakatnik, Limestone cave system in Lakatnik, Bulgaria.

Temnata dupka is a cave in the Lakatnik cliffs, northwest of Gara Lakatnik on the left bank of the Iskar, with its entrance 27 m above the river in a rock wall. It is a system of two main galleries joined by cross passages and set on four floors, cut in Middle Triassic limestone. The longer northeastern gallery runs about 800 m along its main axis and carries the underground river; the west-northwestern gallery is about 710 m long. The galleries are wide and high, mostly rectangular or inverted-trapezoid in cross-section.

The cave system formed millions of years ago when water carved through limestone, gradually creating these passages over time. Its structure has revealed much to scientists about the regional geology and how karst formations developed in Bulgaria.

The cave's older name, Suhodolski podmol, still appears in Bulgarian caving literature. Its lakes carry the names of researchers: Akademik Buresh, after Ivan Buresh, who studied the cave's bats in 1912, and Radevo ezero, after Zheko Radev, who published the first map in 1915. Visitors also pass formations named Chatаl and Furnata.

Exploring requires expert guides and specialized equipment because of the complex passages and technical demands. Visitors should be prepared for uneven surfaces and limited light to safely navigate through the cave system.

The cave's fauna counts 17 stygobiontic species of molluscs, crustaceans and beetles, not 60. Excavations by a Polish-Bulgarian-French mission in the 1980s also found Pleistocene remains of wolf, mammoth, aurochs, reindeer and cave bear, plus Palaeolithic stone tools.