The Abandoned Zeljava Air Base, Croatia

The abandoned Zeljava air base on the Bosnian-Croatian border was Europe's most expensive underground airport tunnel system costing U$6 billion at the time, built between 1948 and 1968. The place was home to 3 MiG squadrons and could withstand a direct hit from a 20-kiloton nuclear bomb. The base was last used in the Balkan wars in the 90's. The airstrip is in a wonderful setting in the mountains surrounded by a beautiful forrest.