The discovery of the first was made by two teenagers playing in a forest, which after some research, I found out was in Northern Germany. One of the boys tripped over two pipes hidden in the undergrowth; after further investigation the boys found an entrance to absent of tunnels hidden since the end of WW2.
On investigation it was found the walls of the tunnels went deep into the forest, deeper than they dared explore. Certain things caused concern for the boys, not least that the walls were charred, and thick metal doors had been blown from their hinges. Inside were a series of mysterious tubes, like periscopes. Connected to these were a series of pipes and dials, the use of these baffled the boys, as did the reason for a room to be swimming in dank water.
I can't give a 100% certain explanation for the state of the tunnels, but as I have worked in the photography trade, and I did some Chemistry in school, I can make some deductions. After some research, I found the area had been a tank training area, as the area was a flat and sandy area during the war years. In the years after the war, the area was left resolute, but enhanced with compost. In the 1970's there was a major explosion in the area. This explosion was the worst fire in the area.
The reason for the explosion was the build up of methane caused by rotting vegetation seeping into the tunnels. The mix with Oxygen in a confined area has to be closely monitored - hence the pipes, and dials - it is a condition known to miners as Marsh Damp, and is highly volatile. Too much methane, and you start the fire you are trying to prevent; too much Oxygen, and you create an explosion.
One of the things I noted was though the tunnel walls had been blackened, they were not charred; indicating the fire was quick, and distant, both indicating a quick fire with a low flash point; such as one from a Meths flash over. I was witness to one in Germany, it was over before we knew it happened, but we weren't in a Methane soaked area.
My research led me to believe the other end of the tunnels led to a storage depot for ammunition, probably for the proposed invasion of Britain. This would fit with the layout of the interior of the tunnels, there were rooms off the main passageway, but none appeared to be for more use than work.
My theory is there was a dangerously high Methane build up, which they tried to vent. Unable to vent enough in time the explosive flash over occurred. The heat from this melted the dials open, releasing too much Oxygen too quickly,causing a huge explosion. The force of which ripped iron doors off their hinges.
Viewing the photos taken, it was plain to see the boys were not the first people to find the tunnels. On the ceiling was painted the universality recognised symbol of the CND movement, which had massive support in Germany in the late 1960's to the middle of the 1970's.
I know so much about mines, because I did some researching for one of my stories -which won't be written- my friend, and editor, Julia thinks I could write a great story on this theme; so do I, but I won't, as nobody would read it.