The cave system of Postojna is really big and complicated. Many caves of the area are connected by sumps, natural passages, or artificial tunnels. Others belong to the same hydrologic system, but have no connection so far. Several entrances have their own name, although they are the same cave, which is a historic remains. The caves were named independently befor their connection was discovered.
From the geologic view, the caves around Postojna are located between two faults. Along those faults the rocks have been lifted, and so the type of rock changes, flysh which is inpermeable on one side and limestone which is karstified on the other side. Rivers flow on the surface on the flysh side and enter caves to flow underground on the limestone side. The flysh is also softer than limestone and thus easier eroded. So the flysh is typuically a low plain and the limestone forms steep ridges.
The biggest cave is obviously Postojnska Jama, with its different parts. The river Pivka enters the cave, flows through the lowest level of the cave, just to vanish in a sump. It reappears some kilometers away in Planinska Jama.