The olm or proteus (Proteus anguinus) is an aquatic salamander in the family Proteidae, the only exclusively cave-dwelling chordate species found in Europe. In contrast to most amphibians, it eats, sleeps, and breeds underwater. Living in caves found in the Dinaric Alps, it is endemic to the waters that flow underground through the extensive limestone of the karst of Central and Southeastern Europe, specifically southern Slovenia, the basin of the Soča River near Trieste, Italy, southwestern Croatia, and Bosnia and Herzegovina.
It is also called the “human fish” by locals because of its fleshy skin color (translated literally from Slovene: človeška ribica,), as well as “cave salamander” or “white salamander”. In Slovenia, it is also known by the name močeril, which translates as “the one that burrows into wetness”. It was first mentioned in 1689 by the local naturalist Valvasor in his Glory of the Duchy of Carniola, who reported that, after heavy rains, the olms were washed up from the underground waters and were believed by local people to be a cave dragon’s offspring.
This animal is most notable for its adaptations to a life of complete darkness in its underground habitat. The olm’s eyes are undeveloped, leaving it blind, while its other senses, particularly those of smell and hearing, are acutely developed. It also lacks any pigmentation in its skin. It has three toes on its forelimbs, but only two toes on its hind feet. It also exhibits neoteny, retaining larval characteristics like external gills into adulthood, like some American amphibians, the axolotl and the mudpuppies (Necturus). The olm is the only species in the genus Proteus and the only European species of the family Proteidae, whose other extant genus is Necturus.