Cave "Temnata Dupka" is the longest Balkan cave (5300 m). The cave entrance is located in the wall of the rock ridge of the Lakatnik (Milanovo) Rocks, about 27 meters above the Iskar River, in the territory of Milanovo village, northwest of Lakatnik station.

It is formed in dark Middle Triassic limestones (Ladinian) of the Milanovo syncline. It represents a system of two main galleries with adjacent branches. The northeastern part of the cave is longer — about 800 meters along the main axis, and the main underground river flows along it.

Usually the underground river flows out through the karst spring "Zhitolub", but during extreme rainfall and snow melting its level rises and the water flows out through the cave entrance. The cave fauna is rich — 18 species of troglobites adapted to life without light have been established. An interesting fact is that Bulgaria is among the countries with the greatest diversity of cave organisms.

Cave "Temnata Dupka" is known as a "school" for Bulgarian cavers and has important significance for the development of speleology in our country.