Cave stamp from Austria

The last two weeks I was on a trip to Austria and Italy. When I was in Austria I was in a post office where I bought some nice Austrian stamps.
One of them is this:

Postojna Cave-Underground Post Office (issued 06-04-2013)
(It is a joint issue with Slovenia)

The Postojna Cave (Slovenian: Postojnska jama; German: Adelsberger Grotte) is located near the city of Postojna in Slovenia and is the second-largest for visitors developed stalactite cave in the world. Between 1900 and 1914 the Royal and Imperial Post had a post office only for the cave