The second-longest cave system in Slovenia is the Postojna Cave which is situated in the south-west of the country. Over the last two million years, the limestone of Postojna has been carved by the Pivka River into multi-level caverns, adorned with stalactites, stalagmites, columns and curtains.

Around five kilometres of the Postojna Cave system is open to the public, stretching from its entrance north-east to Planinsko Polje. Tours begin with a train ride into the cave before proceeding on foot to discover the elaborate formations of its tunnels, passages, galleries and halls with the help of a local guide. Conditions within the cave are quite chilly at 10 degrees Celsius and with humidity levels at almost 100% throughout the year. Postojna Cave is home to an endemic salamander known as an olm or proteus which is the largest troglodytic amphibian in the world. An aquarium within the cave allows visitors to see this unique animal up close and a multimedia presentation explains its life cycle, together with the unique formations seen in the surrounds. Tours are available in a number of different languages, and after around 90 minutes below the surface, the train returns to the entrance where visitors will find a restaurant, an adventure playground and souvenir stalls. In addition to tours, the Postojna Cave is also used as the setting for occasional concerts.

Postojna is connected to the Slovenian capital Ljubljana by both bus and train, with the bus station within walking distance of the cave. During the peak tourist season, a free shuttle operates between the train station and the Postojna Cave, as well as the nearby Predjama Castle.

It was Johann Weikhard von Valvasor who first described the cave during the 17th century, but not until Archduke Ferdinand visited Postojna in 1819 that it officially became a tourist destination. Cave rails for the first tourist train were laid in 1872 and electric lighting added in 1884 before German occupying forces used the cave to store aircraft fuel during World War II.