Magura Cave lies near Rabisha, west of Belogradchik, and is one of north-western Bulgaria's major natural and archaeological sights. Its long limestone galleries are impressive on their own, but the real distinction is the prehistoric wall art, with human figures, animals, and ritual scenes painted in dark pigment.
It is far enough from Belogradchik that you need a car, taxi, or organised transport, but it belongs naturally with a trip to the Belogradchik Rocks. Bring a layer for the cave temperature and check tour access before going, since entry rules can vary by season and conservation needs.