This indicator shows the number of species in subterranean habitats in Slovenia. Only obligate subterranean species, also known as troglobitic species, have been taken into consideration. The figures have been rounded off, as they are constantly on the rise due to advancements in research. The number of species in subterranean waters, i.e. caves, as well as sandy and gravelly alluvia, encompasses approximately 8% of all freshwater fauna in Europe. The share of terrestrial cave fauna cannot be determined.

A considerable predominance of obligate subterranean species over surface immigrants in a subterranean water body also suggests a high level of cleanliness and thereby usefulness of the subterranean water.

Slovenia has an exceptional number of obligate subterranean species. It seems that aquatic fauna, with 200 species, is by far the richest, while terrestrial fauna, with 150 species, is second only to more southern parts of the Dinaric karst. Roughly 55% of aquatic fauna is comprised of tiny crustaceans, while 25% are small snails. Over 50% of terrestrial species are beetles. The highest density of species is found in the Dinaric karst of southern Slovenia and other karst areas, while some aquatic fauna is also found in groundwater in non-karst lowlands.

The number of obligate subterranean species in individual cave systems is also high. As many as five Slovenian cave systems have been listed among the 20 richest cave systems in the world. The Postojna-Planina cave system, with its 50 water and 35 terrestrial species is by far the most abundant. Another advantage of this system is the plethora of "young" surface immigrants in the underground river and entrance passages. Species that are progressively better adjusted to the subterranean world as we move along the underground river are of special interest. Apart from caves, a very high number of species are also found in the water of the gravelly lowlands of the Ljubljana Plain (Ljubljansko polje).

Most subterranean animals are not threatened by collectors (scientists or merchants). They are, however, all threatened by pollution, which normally originates from the surface. Many substances pose a direct threat to the underground life. The input of seemingly less dangerous organic substances into the underground enables the invasion of surface species, which are driving out obligate subterranean species. Water in the underground purifies itself only to the level of nitrates, which then accumulate. For this reason, the proteus and subterranean beetles require and are given special protection by the Decree on protected wild animal species (Official Gazette of the Republic of Slovenia, Nos. 46/2004, 109/2004, 84/2005, 115/2007 and 32/08-decCC). This decree also protects the habitats of other subterranean species, i.e. the subterranean environment as a whole. Apart from that, a substantial part of especially biodiverse areas is partly under Natura 2000 protection, while only the rather modest subterranean fauna of the Škocjan Caves is under special protection (as a Ramsar wetland).

The state of waters in sandy and gravelly deposits below the plains is deteriorating; there are also fewer and fewer usable wells where monitoring the conditions is still simple. The state of these deposits has deteriorated significantly in certain places in karst areas as well. The invasion of surface species along the underground river was observed in the Postojna-Planina cave system in the post-war period, but after remediation, the situation has improved. The state of deposits in the Kočevje area is extremely bad. Some subterranean waters are polluted due to intensive livestock breeding and others, due to landfills; the once exceptionally dense proteus colonies have since disappeared. Similar phenomena can be observed elsewhere, though to a smaller extent; animals previously present in caves that are not particularly polluted often cannot be found anymore. There is no systematic monitoring of these changes.

The vast majority of obligate subterranean species are endemic. This means they are only limited to a certain place or area. They are mostly limited to the southern half of Slovenia and many of them to a much narrower area. In addition, their general habitats are broken up into many small patches, making all of these species threatened.