We’re not sure whether this is the largest abandoned underground military base in Europe, but we’re absolutely certain that it is one of the most intriguing. Located on the border between Bosnia and Croatia, Željava Air Base was built in 1968 and was the largest in what was then Yugoslavia, not to mention one of the most expensive military bases in Europe. Yugoslavia’s demise means it is now entirely abandoned, buildings left to rot and huge underground tunnels neglected to the whims of nature. There are a number of landmines left over here from the last war, but these areas are clearly marked. A rotting Douglas C-47 stands at the entrance, a curious image in itself.