The cave was first described in the 17th century by the pioneer of study of karst phenomena, Johann Weikhard von Valvasor, although graffiti inside dated to 1213 indicates a much longer history of use.[1][2] In 1818, when the cave was being prepared for a visit by Francis I, the first Emperor of the Austria-Hungary, a new area of the cave was discovered accidentally by Luka Čeč, a local man in charge of lighting lamps in the cave. In the 1850s, the Austrian-Czech geographer Adolf Schmidl published the first comprehensive scientific overview of the Postojna caves and the Pivka Basin, which became a standard reference point in the study of speleology.[3]

First tourist guide and electric lighting

In 1819, Archduke Ferdinand visited the caves, this is when the caves became officially known as a tourist destination. Čeč became the first official tourist guide for the caves when the caves were opened to the public. Electric lighting was added in 1884, preceding even Ljubljana, the capital of Carniola, of which the cave was part at the time, and further enhancing the cave system's popularity.

In 1872, cave rails were laid along with first cave train for tourists. At first, these were pushed along by the guides themselves, later at the beginning of the 20th century a gas locomotive was introduced.

During World War I, Russian prisoners of war were forced to construct a bridge across a large chasm inside the cave.[1]

Mid-century changes  Tito and Sukarno at the cave, 1960

During World War II, German occupying forces used the cave to store nearly 1,000 barrels of aircraft fuel, which were destroyed in April 1944 by Slovene Partisans. The fire burned for seven days, destroying a large section of the cave and blackening the entrance.[4][5][6]

After 1945, the gas locomotive was replaced by an electric one. About 5.3 kilometres (3.3 mi) of the cave system are open to the public.[7]

At the end of the 1990s it was one of world's most visited show caves, with nearly 1 million tourists per year.[8]

21st-century tourism

In June 2015 and May 2017, the cave administration reported that cave divers managed to explore a further underwater section of the cave leading towards Planina Cave, thus lengthening the cave system from 20,570 m (67,490 ft) to 24,340 m (79,860 ft).[9][10] The cave also houses the world’s only underground post office.[11]