Prohodna is the best known attraction in the Karlukovo Gorge (part of the Geological Park Iskar-Panega), one of the largest karst regions in Bulgaria and a popular location for speleology.[1] Formed during the Quaternary, Prohodna is 262 metres (860 ft) long, which makes it the longest cave passage in Bulgaria. The cave has two entrances which lie opposite one another, known respectively as the Small Entrance and the Big Entrance. The former is 35 metres (115 ft) high and the latter reaches 42.5 or 45 metres (139 or 148 ft)[2] in height.