This reminds me of the tunnels in Seiglesbach Germany where i served as an MP. It was an old V2 missile base. during WWII. Many of the tunnels were closed up and sealed as were the old Nazi bunkers. I remember the bunkers had swastika in the bricks fronts. Some of the tunnels were accessible and only partially flooded in summer and the seals had broken. I was rumored a tunnel big enough to drive a regular 2.5 ton truck existed from there to Heilbronn. Even my German friends knew about the tunnels and bunkers on our base. It was forbidden to get caught in them. Any artifacts soldiers found were considered contraband and to be turned in to the armory. Some of us did go into a tunnel there but it was was mostly flooded and rat infested at that time. the whole underground network was flooded and sealed up supposedly but I knew the dog handlers there had found several tunnels they used at night.