Embark on the Dark Worlds walking tour and be transported to a fascinating time in Berlin's history. Hundreds of people a day walk past a green door in the Berlin subway station Gesundbrunnen without knowing that behind it, rooms entrenched in history exist. How uncomfortable it must have been to have been crammed inside these small rooms, with the constant whirr of the ventilation machine and the terrible thundering of the bomber aircraft overhead. The atmosphere can still be sensed today.