Zeljava Airbase

Željava Air Base, situated on the border between Croatia and Bosnia and Herzegovina under Plješevica Mountain, near the city of Bihać, Bosnia, was the largest underground airport and military air base in the former Yugoslavia, and one of the largest in Europe. The facilities are shared by the local governments of Bihać and Lika-Senj County in Croatia.

The site is abandoned and in decay now, but easily accessible by motorcycle. The underground bunkers can be explored but care has to be taken as there are no lights or safety measures present. 
As the runways cross a border (Croatia and Bosnia), it's illegal to ride the whole length. Take note of all signs for border crossings and mines, they are there for a reason.

https://en.wikipedia.org/wiki/%C5%BDeljava_Air_Base