If you're not a fan of cave fauna, Vivarium might not appeal to you, especially since its main attraction - the olm - can also be seen in an aquarium during the Postojna Cave tour. However, if you decide to visit, perhaps taking advantage of the discounted admission as part of a combo ticket with the caves and the Predjama Castle, you'll experience a half-hour tour of a cave that once served as a research laboratory. In the cave, you can observe local fauna in terrariums and aquariums. Unfortunatelly, many of the creatures are so tiny that they're difficult to spot. Be prepared for the cave's temperature, which is around 10°C. The entire exhibit is very well done, with informative panels and an audio guide available in multiple languages, which I particularly appreciated.