Postojna Cave is a 24,340 m long karst cave system near Postojna, southwestern Slovenia. It is the second-longest cave system in the country (following the Migovec System) as well as one of its top tourism sites.
Postojna Cave houses the first underground post office building in the world. The first cave post office in the world was opened eighty years after discovering the most beautiful parts of Postojna Cave. It opened its door to cave visitors in 1899, at a time when postcards were all the rage.
The underground post office catered only to cave visitors and guests at special events, the so-called Cave Holidays in the famous Dance Hall. Right on the edge of the Dance Hall, about 500 meters from the entrance to the cave, was a small stone post office (4 x 2 meters in size) with a door and a window opening, where visitors could buy and send postcards. Since Slovenia was at the time part of the Austro-Hungarian Empire, the post office was Austrian.
The post office was always open on Whit Monday, as well as on other days of the year, but not necessarily every day. The fact that the post office was a must-see part of the Postojna Cave tour speaks volumes about how important sending postcards was at the time. On their way to the inside of the cave and back, cave trains would (during a certain period in the past) stop right in the Dance Hall, near the post office, so visitors could buy and send the cave postcards.
The number of postcards posted in 1900 amounted to 7,000, while four years later, on Whit Monday, no fewer than 13,800 postcards were sold in a mere three hours, each with a special postmark.
According to the Internet