Posta Slovenije issued a joint commemorative block with Austria entitled "Postojna cave, cave post office 1899".
Postojna Cave, the first underground post office, is the most famous cave in Europe, welcoming half a million visitors each year. In its 200-year tradition of tourism, the cave's beautiful dripstone formations have been admired by 35 million people, including 150 monarchs. The cave extends for 21 kilometers. Tours of the cave have been possible for 141 years via the cave railway. It is a cradle of biospeleology, and an area with the greatest diversity of cave fauna in the world. Prior to the appearance of postcards, the cave had already enjoyed a long tradition of tourism.
The number of postcards sent from Postojna grew markedly at Whitsuntide and the Feast of the Assumption, when mass events and dances were held. For this reason the cave management sent a request to the Austrian trade ministry in Vienna for the opening of a post office in the cave. On 15 August 1899, next to the Dance Hall, 500 meters from the entrance to the cave, the first underground post office in the world was opened. At first the post office was open only on special occasions, but after 1911 it operated regularly, with a record 75,000 postcards being sent at that time on Whitsun Monday, and thereafter 6,000 to 11,000 were posted each day.
Even during the First World War the underground post office stayed open, and continued to operate between 1922 and 1927, after the annexation of Postojna to the Kingdom of Italy. In May 1927 a second underground post office was constructed in the Concert Hall, 1400 meters from the cave entrance, and this operated up until 1945.