According to explorers' estimates, the Postojna Cave provides shelter to more than 150 animal species and visitors can get to know some of the most interesting ones in more detail in the Vivarium Proteus.
The Postojna Cave is the cradle of a special branch of biology# called speleobiology. This relatively new science was born in the year 1831, when the Slendernecked Beetle (Leptodirus Hochenwarti) was discovered in the Postojna Cave by Luka Čeč, a local of Postojna. Not long after, explorers discovered and made an inventory of scores of new animal species adapted to the conditions inside the cave to such an extent that they are unable to survive outside of it.

The Vivarium Proteus is located inside the Passage of New Signatures, 50m from the entrance of to the cave. It consists of two sections, the research section and the exhibition one. The research section, the so-called laboratory, is used for scientific work and research. And the speleobiological exhibition housed within the exhibition section introduces visitors to the basic concepts of karstology and speleobiology in an appealing way, and presents special features of the subterranean environment and the most characteristic representative of Slovene cave animals. Most attention is however devoted to the human fish, also known as the olm or Proteus Anguinus.

Capable of growing up to 30cm in length, the human fish is the largest cave-dwelling animal and is the only vertebrate in Europe that lives solely in the subterranean world. Extremely well-adapted to life in the darkness, its skin lacks any protective pigmentation and the blood showing through it makes it very pale or slightly pink in colour. It was given the name human fish because of the colour of its skin, which resembles the skin of people. Its eyes are undeveloped.

In Slovenia, the human fish is under strict legal protection and was in 1982 placed on a list of rare and endangered species. It's forbidden to catch them without special consent.