In the same year that Parolini explored the cave, he introduced, from Postojna in Slovenia, Proteus anguinus, or olm, a very slow-moving, blind cave-dwelling amphibian, about 15 cm long, pink in colour and with permanent gills. For more than a century and a half the experiment was thought to have failed and Brentari actually wrote that they had died out. Instead, in 1965, the Friuli Speleology and Water Group found a few specimens in the underwater sections of both Covol dei Siori and Covol dei Veci, sightings that were then regularly repeated by many cave-divers.
Besides Proteus and a few fish that swim in the Brenta river, the hypogeum environments host tiny creatures that have undergone the necessary adaptations to live in cave environments, the most common of these being insects like Troglobites, Troglophiles and Trogloxenes.
Outside of the caves, the particular natural environment favours the presence of the European dipper (cinclus cinclus) which has the specific ability to walk along the river bed against the current using its wings like spoilers to keep it down on the river bed – hunting insect larvae and small crustaceans that it winkles out with its sharp beak. There are plenty of other animals that frequent the source springs of the Oliero river, common to all wetland areas in the valley.