The old town core of Kranj is situated on a conglomerate prominence (from Pleistocene) between the rivers Sava and Kokra. The conglomerate is well agglutinated in some spots, and poorly agglutinated in other spots as well as gravely sandy. The gravel stones were bound into conglomerate during the warmer interglacial periods. Due to favourable climatic conditions the lime binder glued the gravel stones together. The later rising and falling of underground waters caused dissolution of the lime binder, karstification and forming of cracks, grottoes, caverns, caves and even rock falls. Similar processes are still going on today. The tunnels under Kranj and its surroundings have been caved into Pleistocene conglomerates. The conglomerate walls can be clearly seen in the places without concrete coating. All manmade tunnels were dug during World War II, namely for the purpose of shelters and dugouts for the civilian population.

There are numerous manmade underground constructions under Kranj, which ceased to serve their purposes long ago and have been forgotten. One of the most interesting among them is the near 1.300 metres long old city bunker, being an important monument of construction building and technical culture of Kranj during World War II. The bunker has four accesses – from the slope Jelenov klanec, from the Kokra canyon in Lajh, and along the staircase from the back of the Prešeren’s Theatre. The tunnel starting at the slope Jelenov klanec crosses a conglomerate prominence and then opens up into the protected natural monument of the Kokra River canyon. A long southern extension branches off under the Post and leads all the way to Lajh. All tunnels are of approximately equal dimensions, 2 metres wide, only the section under Pungert is slightly branched out. They are fortified by a layer of concrete, the unfinished section showing the original conglomerate dugout. Apart from one smaller chamber there are no larger rooms. The Kranj community started building already on the eve of World War II. The work was newly started during the occupation in 1944, when the danger of allied forces bombing from Italy increased. The works on tunnels were performed by two construction companies from Kranj – Josip Slavec (known for construction of bridges and high-speed roads before the war, constructed the slope Gaštejski klanec) and Josip Dedek (known for construction of dams for hydroelectric power stations). The majority of construction works was finished by the end of 1944, performed by 130 hired workers, but there was not enough material and time to finish the protective concrete coating and other installations. The bunker was nevertheless frequently used by the citizens during the last few months of World War II, when there were frequent air defence warnings because of the allied ferry flights. After the war the bunker was abandoned, but became of interest for a short period again during the Slovene independence war in 1991 – the Kranj civil protection unit arranged the entrances and restored the lightning installation.

Tular is a natural cave, which was dug into the Sava conglomerates by a small brook. It was first mentioned already in 1689 by the famous naturalist J.V. Valvasor. The fauna of this cave is very interesting, if we mention only the endemic subspecies of the small cave beetle (Anophthalmus miklitzi ssp. staudacheri), and probably also the subspecies of the blind cave crustacean (Niphargus ilidžensis ssp. slovenicus). In 1944 it was partly walled into an air-raid defence shelter for the nearby factory. In 1960 the abandoned shelter was turned into a cave laboratory by Marko Aljančič (1933 – 2007), who inhabited it with the European blind cave salamander (Proteus anguineus). It is the only cave laboratory in Slovenia and – apart from the cave laboratory in Moulis in France – the only place to breed Proteus anguineus out of nature. In the laboratory they study the ecology and ethologic of the blind cave salamander, mainly it’s breeding. Apart from that they also work on raising the public awareness of the blind cave salamander as the symbol of the Slovene natural science, with special emphasis on nature protection. Since visitors could disturb the animals, the laboratory is not open for public.   Not many people know Kranj and its surroundings due to mining, but the hills of Šmarjetna gora and St. Jošt were known to the searchers of the iron ore as well as the manganese in the second half of the 19th century. The last mining attempts date in the not so far-off year 1924. At the end of the 19th and the beginning of the 20th century there was a smaller mine in Gorenja Sava at the foothills of Šmarjetna gora, which supplied manganese to the Jesenice ironworks. With the invention of a revolutionary steel fortifying process with manganese in 1872, which changed the steel industry worldwide, the mine has become a part of the steel working history. The almost 400 metres long mine level has two entrances. The tunnels are up to 5 metres wide. They were equipped with tracks for the pit carts, which were pulled by men or horses. Miners cut through numerous smaller natural caves – abysses in the limestone of Šmarjetna gora, and cave animals moved from the caves into the mine levels, among them also the endangered species of a cave beetle (Anophthalmus besnicensis), an endemic beetle from the surrounding of the hill Rovnik above Besnica near Kranj.

During World War II Germans built more open fir fighting water tanks in Kranj, which have later mostly been filled up. On the location of the city fountain (since 1955) on Main Square a large underground water tank for fire fighting was built in 1943 by the construction company Josip Dedek. It was first filled up with water on 16th January, 1944. The tank, which is completely dug in, has two narrow open shafts next to the fountain. It is 3,5 metres deep, 4 metres wide and 30 metres long. It contains 310 cubic metres of clean water and could therefore still serve its purposes. After the war the Kranj firemen maintained the tank for some time, but due to the modern technical equipment it has later fallen into oblivion. In 2002 it was researched and documented anew by members of the Unit for Rescuing from Caves and Heights (JD Carnium) and the Unit for Rescuing from and on Water (the Society for Underwater Activities Kranj). Since the underground water tank is filled to the top, it can be compared to a siphon and is therefore accessible only for the most experienced cave divers. Since it has been inhabited by the blind cave crustacean (Niphargus ilidžensis ssp. slovenicus), it bears some resemblance to natural caves.

The abandoned tunnels, where darkness, moisture and constant temperature create an environment similar to a natural cave, have been inhabited by animals that are not typical cave animals, but enjoy the conditions there. The most frequent, especially in the entrance sectors, are the cave cricket (Troglophilus cavicola) and the cave spider (Meta menardi). Rare but very interesting inhabitants are the bats. Visitors should observe them from distance and without too much noise, especially in winter time, since bats need a lot of energy for waking up from winter sleep, which may endanger their survival until spring. In the dusk of summer evening bats fly out of tunnels to feed on insects. The most common is the lesser horseshoe bat (Rhinolophus hipposideros) with up to 10 centimetres, while the greater horseshoe bat (Rhinolophus ferrumequinum) with up to 15 centimetres is much rarer. While resting they both hang with their heads down and their wings enfolding the body.

On spots with water dripping stalactites are formed, which are the same as in natural caves. They are built of calcite crystals, which is precipitated from mineralized water solutions that slowly trickles through the 15 metres thick conglomerate ceiling above the tunnels. Stalactites are narrow hollow tubes, known also as “soda straw”, which are very fragile and should therefore not be touched. The thicker solid stalagmites slowly rise from the floor. In winter the dripping rainwater near the entrances occasionally freezes and forms seasonal ice stalactites.

In the 80-ties and 90-ties of the previous century the abandoned tunnels under the slope Jelenov klanec and Pungert were used as cultivation ground for champignons – mushrooms that need darkness and moisture to grow. Unfortunately all the dirtiness was left in the tunnels after the growing of mushrooms had been given up. In 2003 the Speleological Society of Kranj started removing the rubbish. In 2005 they have cleaned most of it in the tunnels under the slope Jelenov klanec, and in 2007 also at the entrances to the Kokra River canyon. Work was resumed by the Kranj Civil Protection Units: Unit for Rescuing from Caves and Heights (the Cave Exploration Society Carnium), Unit for Temporary Accommodation of Inhabitants (Scout Association of Kranj), Unit for Rescuing from and on Water (the Society for Underwater Activities Kranj), Unit of Rescue Dog Handlers (the Kranj Rescue Dog Handlers Club), and Signal Unit (Radio Club Kranj). During 2006 and 2007 they removed more than 150 cubic metres of rubbish from the tunnels. The work was performed by volunteers within the sphere of activities for the yearly action “Let’s Clean Kranj”, organized by the Scout Association of Kranj. With their work they have paved the way to revival of the abandoned underground constructions. The Municipality of Kranj, together with the Tourist Board Kranj, the Institute for Protection of Cultural Heritage, and the Nature Protection Institute of the Republic of Slovenia, has decided to tidy and put in order the tunnels and the entrance platforms. Therefore the pavements and electric lighting are being arranged as well as the platforms on the Kokra River side, which can be accessed from Pungert and Jelenov klanec. The total investment will amount to EUR ……… The Tourist Board Kranj will be responsible for contents and exhibitions in the tunnels.