Svinska Dupka (Pig's Hole) cave is located 2.2 km northwest of Lakatnik station at the base of a rock wall along the Petrenska River. Its triangular entrance (6.85m wide, 2.70m high) leads into a semi-lit vestibule with signs of past archaeological excavations. A straight 140-meter gallery (up to 3.5m high and 8m wide) leads to a zig-zagging section ending in a chamber with wet clay and fallen rock blocks. The cave is warm, humid, and serves as a year-round bat refuge.

The cave hosts Paralola buresi, a rare arachnid species described in 1951, previously thought endemic to Lakatnik. Excavations also uncovered numerous cave bear remains, making it a true prehistoric "bear tomb."

The name stems from a local legend about a swineherd who lost his pigs here and found them all the way at Ledenika Cave near Vratsa—a common Bulgarian folklore motif involving subterranean passages spanning vast distances. Another legend claims locals used torches to navigate the cave over 1–2 days to reach markets in Vratsa safely during the Ottoman era, until a rockslide blocked the underground path.