Cave Post office in Postojna Cave, Slovenia

Postojna Cave is the most famous cave in Europe, welcoming half a million visitors each year. It also hosted an Underground Post office inside the cave. Postojna cave was visited by thousands of tourists, and therefore, 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.

A Special Commemorative M.S. was issued by Posta Slovenije as Postojna Cave,  cave post office 1899  on new Slovenia-Austria joint stamp issue on 22.03.2013