Hungary - Slovakia - Caves of Aggtelek Karst and Slovak Karst

"The variety of formations and the fact that they are concentrated in a restricted area means that the 712 caves currently identified make up a typical temperate-zone karstic system. Because they display an extremely rare combination of tropical and glacial climatic effects, they make it possible to study geological history over tens of millions of years." 

Source: UNESCO World Heritage

Postcard 1 - Hungary
Baradla Cave of Aggtelek Karst in north east of Hungary at the Slovakian border. Nice stamps used. Thanks to Rita of Hungary.

Postcard 2A - Slovakia
Dobsina Ice Cave, a UNESCO World Heritage site in Slovakia. Thanks to Rita for mailing from Slovakia.

Postcard 2B - Slovakia
Slovakia Caves of Ochtinska aragonitova jaskyna. Thanks to Radek of Czech Republic.