I've been.
http://berliner-unterwelten.de/home.1.1.html
They're open to the public and used for tours. Right now they're in the process of selling most of them and they'll be converted into businesses. Night-clubs and such. So it's the last chance to see many of them. After WW2 they were converted into nuclear bunkers and have that 60'ies futuristic look about them. Hitler's bunker was filled up after the war. But the lower levels are still intact I think. They're in the process of excavating them now I think.
Berlin has a problem of being built on what basically is a swamp, with very high groundwater making maintaining bunkers impractical. It's really the worst place to have anything underground. This means that if there's not plenty of money in maintaining them they won't, and they'll flood. At the height of the cold war, despite their best efforts, West Berlin had nuclear bunkers for about 3% of the west Berlin population. This can be compared to Stockholm which had bunkers for 100% of it's population. Stockholm is built on solid granite. This is what they told me on the tour.
But just make shit up. Nobody is going to care if it's realistic or not. In everybody's imagination there's always hidden passages below filled with monsters. Go for it.