For whatever reason, I just ended up reading about Berlin's Tempelhof airport on Wikipedia through a series of random links as often happens on that website. This blurb at the end of the WWII history section caught my eye-
"Russian troops attempted to clear the five lower levels of the airbase, but the Germans had booby-trapped everything and many were killed; leading the Russian commander to order the lower levels be flooded with water. The lower three levels are still flooded to this day, having never been opened up due to unexploded ordnance."
It is Wikipedia, so I'm not so sure this is 100% accurate. I've done some basic googling and such, trying to find details of the underground levels of the airport and whether there really are 3 levels of rooms in a perfect state of preservation from 1945, minus the bombs planted by Germans and the water of course. That would be fascinating, but so far I've only run into dead ends and reports that mention booby traps but are no more concrete than the Wiki reference... any of our Berlin residents have any input?