Postojna Cave is the second largest cave in Slovenia, and one of the oldest natural caves in Europe. It was formed nearly a million years ago, and its formation is due to many natural factors, including the passage of the Pivka River through it, and the river is still passing through it until the present time. The cave extends underground for a distance of 25 km, and contains many types and colors of stones that were formed and acquired their colors over the centuries due to various environmental factors. The trip inside the cave is by an open train, but there is a specific area of the cave that is to be walked around.