The discovery of the olm, or the human fish, dates back to the beginnings of the  exploration of the underground world. When Janez Vajkard Valvasor wrote of dragon hatchlings of Vrhnika in 1689, he had no idea that he was in fact describing a previously unknown species. It was only later, in 1768, that J. N. Laurenti provided a scientific description of the new animal and named it Proteus anguinus.

At 25 to 30 cm, the olm is the largest cave animal in the world. Apart from its size, it is a typical troglobiont, lacking pigmentation and eyes. The latter are present in larvae, but are later overgrown by skin. Nonetheless, through a special mechanism called dermatoptic sensing, it can still detect light with its skin. It has very good senses of smell and taste, and its inner ear is unusually well developed. The olm also possesses a unique sense for detecting electric fields. The locomotion function of the two pairs of weak legs, which are far apart and have three toes on the front and two toes on the back pair, is auxiliary at best. It breathes through external gills, which are filled with blood and brightly red due to skin translucency. In addition, it has simple lungs. The sexes differ only slightly in appearance.

There are two subspecies living in Slovenia: the white olm (“Proteus anguinus anguinus”, discovered by Laurenti in 1768) and the black olm (“Proteus anguinus parkelj”, discovered by Sket & Arnzten in 1994). The black olm is only found in Bela krajina. There are several differences between the black and the white variant. Obviously, the black olm has darkly pigmented skin, but it also has eyes which its white relative lacks, shorter snout and wider head, shorter legs and tail, and a longer body.