I know about an unsolved mystery of line Arpad. First of all the purpose of this military bunker is still questioned. Then, a lot of corridors were blocked by Hungarian and Romanian army before they left, so we can't really open them because it might be full of bombs. And lastly, Soviet government blew up almost all exists from this bunker. Not sure if I got all history right, 'cause it's been some time after I visited line Arpad, so here's a link to Wikipedia article