Living in caves found in the Dinaric Alps, it is endemic to the waters that flow underground through the extensive limestone bedrock of the karst of Central and Southeastern Europe in the basin of the Soča River (Italian: Isonzo) near Trieste, Italy, southern Slovenia, southwestern Croatia, and Bosnia and Herzegovina.

It exhibits neoteny, retaining larval characteristics like external gills into adulthood,[5] like some American amphibians, the axolotl and the mudpuppies (Necturus).

[11][12] The olm's body is snakelike, 20–30 cm (8–12 in) long, with some specimens reaching up to 40 centimetres (16 in), which makes them some of the largest cave-dwelling animals in the world.

[15] Females grow larger than males, but otherwise the primary external difference between the sexes is in the cloaca region (shape and size) when breeding.

Its body is covered by a thin layer of skin, which contains very little of the pigment riboflavin,[16] making it yellowish-white or pink in color.

It retains larval proportions, like a long, slender body and a large, flattened head, and is thus able to carry a larger number of sensory receptors.

[26] The taste buds are in the mucous epithelium of the mouth, most of them on the upper side of the tongue and on the entrance to the gill cavities.

[27] The sensory epithelia of the inner ear are very specifically differentiated, enabling the olm to receive sound waves in the water, as well as vibrations from the ground.

[28][29] As this animal stays neotenic throughout its long life span, it is only occasionally exposed to normal adult hearing in air, which is probably also possible for Proteus as in most salamanders.

[39][40] Sexually mature males have swollen cloacas, brighter skin color, two lines at the side of the tail, and slightly curled fins.

[43] Unconfirmed historical observations of viviparity exist, but it has been shown that the females possess a gland that produces the egg casing, similar to those of fish and egg-laying amphibians.

[39] Paul Kammerer reported that female olm gave birth to live young in water at or below 13 °C (55 °F) and laid eggs at higher,[8] but rigorous observations have not confirmed that.

[44] Development of the olm and other troglobite amphibians is characterized by heterochrony – the animal does not undergo metamorphosis and instead retains larval features.

The thyroid is normally developed and functioning in the olm, so the lack of metamorphosis is due to the unresponsiveness of key tissues to thyroxine.

Earlier researchers used these differences to support the division into five species, while modern herpetologists understand that external morphology is not reliable for amphibian systematics and can be extremely variable, depending on nourishment, illness, and other factors; even varying among individuals in a single population.

[50] In his 1998 book, J. Alan Hollman described the species as a "problematic" taxon, saying that Brunner's drawing of the bone does not adequately show the differences between P. bavaricus and P.

[50] The first written mention of the olm is in Johann Weikhard von Valvasor's The Glory of the Duchy of Carniola (1689) as a baby dragon.

In his book Valvasor compiled the local Slovenian folk stories and pieced together the rich mythology of the creature and documented observations of the olm as "Barely a span long, akin to a lizard, in short, a worm and vermin of which there are many hereabouts".

It was not until the end of the century that Carl Franz Anton Ritter von Schreibers from the Naturhistorisches Museum of Vienna started to look into this animal's anatomy.

Soon, the olm started to gain wide recognition and attract significant attention, resulting in thousands of animals being sent to researchers and collectors worldwide.

A Dr Edwards was quoted in a book of 1839 as believing that "...the Proteus Anguinis is the first stage of an animal prevented from growing to perfection by inhabiting the subterraneous waters of Carniola.

She learned that they detected prey's motion, panicked when a heavy object was dropped near their habitat, and developed color if exposed to weak light for a few hours a day, but could not cause them to change to a land-dwelling adult form, as she and others had done with axolotl.

[54][55] Additionally, there is evidence that a small number of olms were introduced to the United Kingdom in the 1940s, although it's highly likely that the animals perished shortly after being released.

[56] The olm was used by Charles Darwin in his seminal work On the Origin of Species as an example for the reduction of structures through disuse:[57] Far from feeling surprise that some of the cave-animals should be very anomalous...as is the case with blind Proteus with reference to the reptiles of Europe, I am only surprised that more wrecks of ancient life have not been preserved, owing to the less severe competition to which the scanty inhabitants of these dark abodes will have been exposed.An olm (Proteus) genome project is currently underway by the University of Ljubljana and BGI.

[59] The contamination of the karst underground waters is due to the large number of waste disposal sites leached by rainwater, as well as to the accidental overflow of various liquids.

[60] Among the most serious chemical pollutants are chlorinated hydrocarbon pesticides, fertilizers, polychlorinated biphenyls (PCBs), which are or were used in a variety of industrial processes and in the manufacture of many kinds of materials; and metals such as mercury, lead, cadmium, and arsenic.

Annex IV additionally lists "animal and plant species of community interest in need of strict protection", although this has little legal ramifications.

After joining the European Union in 2004, Slovenia had to establish mechanisms for protection of the species included in the EU Habitats Directive.

The olm is included in a Slovenian Red list of endangered species, thus its capturing or killing is allowed only under specific circumstances determined by the local authorities (e.g. scientific study).

Since the 1980s until the most recent assessment in 2022 the organisation has rated the conservation status for the IUCN Red List as 'vulnerable', this because of its natural distribution being fragmented over a number of cave systems as opposed to being continuous, and what they consider a decline in extent and quality of its habitat, which they assume means that the population has been decreasing for the last 40 years.