Postojna Cave from Slovenia

Postojna Cave was discovered in 1818 by Luka Čeč and opened for visitors in 1819. The caves were created by the Pivka River and are a 21km long underworld system. Nowadays it hosts a natural Concert Hall as well. The stamp was issued in March 2013 by Posta Slovenije. It is a minisheet that commemorates the first underground post office in the world, opened on 15th August 1899 to handle the large volume of postcards sent from the caves.

Castle Gewerkenegg from Slovenia

Castle Gewerkenegg dominates the town of Idrija in Slovenia. It was built in 1527 to host the administrative offices and warehouse of the Idrija mine of quicksilver, the second largest mercury mine in the world. It has been renovated in the last 20 years and today there are a museum and a music school inside its walls. The stamp was issued by Pošta Slovenije in 2004 to celebrate the castles and manors of the country.