Postojna Cave is a 24,340 m long karst cave system near Postojna, southwestern Slovenia. Postojna Cave was carved by the Pivka River over millions of years. There are stalagmites, stalactites, and formations called curtains or draperies that look like folded curtains.
The caves are also home to the endemic olm, the largest troglodytic amphibian in the world. The tour through the caves includes an aquarium with some olms in it. On January 30, 2016, a female olm at the cave began to lay over 50 eggs. This rare event led to global news about Postojna Cave and the olm. From the end of May to mid-July 2016, twenty-two baby olms successfully hatched.
More info: youtube.com