I'm on a little roadtrip in Croatia with Eelco and visit abandoned military relics throughout the country. On the last day we visit Zeljava air base, which was the largest underground airport of Europe. It's located right on the border of Croatia and Bosnia and Herzegovina. This place became operational in 1968 and got destroyed by the Serbian military in 1991-1992. Since then it's a dangerous abandoned place since there are still mines and unexploded ammunition scattered around. This place is also a smuggling route and a illegal way into the EU for illegal immigrants so there's a lot of police patrol. We also visit a "submarine" tunnel called "Hitler's eyes'.
