You wouldn’t give the two green steel doors any thought as you climb the stairs at Gesundbrunnen U-Bahn (underground) station in Berlin. If you even noticed them, you might simply think that they were to provide access to the workings of the underground railway system. But thanks to the Berliner Unterwelten Association (Berlin Underworld Association), a non-profit organisation made up of people from all walks of life, the mystery of what lies behind these innocuous doors is revealed. And as is often the case, looks can be more than deceiving. As you enter what was the airlock (whilst gas had effectively been outlawed, both sides possessed such weapons, and there was of course the danger of fire) of the Gesundbrunnen bunker, you are taking a step back more than 60 years into some of the darkest days of the 20th Century.