I lived on Fliegerhorst as an Army brat from 1972-1974. We all heard the same story from a number of sources.
According to the tales, there are 7 or 8 underground levels. During WWII, the Germans would flood the airfield so that it would look like a lake when Allied bombers flew over, thereby confusing the crews. After the bombers would pass, the "lake" would be drained and the aircraft, stored in the underground hangers, would be taken out. When I was there we were told by our fathers, the Base CO, etc. that only the first 3 levels were "open", and that the bottom 5 levels were "flooded". I know as a kid we explored the open levels, and had our Boy Scout meetings in one of the rooms, guarded by a metal, gasketed door with tog locks. There was also a gun range underground. We came across a huge metal door which was welded shut; this, we were told, led to the flooded levels, and that the Army had sealed it off. The reason?
Apparently towards the end of the war, the Germans had taken all their equipment down to the lower levels, airplanes included. They then booby-trapped the lower levels and opening the water pipe / valve which was used to flood the airport, flooded the lower levels. The Army had tried repeatedly to pump the water out, but were unsuccessful. Apparently the water came in as fast as they tried to pump it out. They speculate that a pipe runs to some water source (I heard "Rhine" and "North Sea", but I doubt the latter). Unable to pinpoint how the water was coming in, they sealed the levels off to keep people (like us kids) from going in, and due to the presence of booby traps. We were also told that the last German who knew how the water was coming in, as well as the locations and types of boobytraps, died in the late 50's with American Intelligence still questioning him. He never gave them the answers.
I know the levels were underneath the old military hangers (as stated, we played and had meetings in them.) The story given above was consistent everywhere we went. Personally I would love to get an ROV, reopen the old levels, and see what lays underwater, and if the source could be blocked. Apparently the Germans had "mothballed" a lot of their equipment prior to flooding in anticipation of returning to use it. After this many years I doubt the Stukas and whatnot would be in recognizable condition, however, if the door we used to attend Scouts was any indication, and knowing German thoroughness, I would strongly suspect there are sealed rooms in the flooded levels which contain well preserved mothballed gear and weaponary items. It would be well worth an expedition to open up and explore those flooded levels; I have no doubt gear is there, and given today's modern ROV technology, I think it could be done in a safe and reasonable way. There's no telling what you might find down there! (I wouldn't doubt you might find some old dead Germans sitting in waterproof rooms -- waiting for their buddies to return!)