The guide leading our group on this tour of Berlin's unseen history apologises for the smell. There is a lot down here that people above would rather not know about, including the sewage canals.
There are though, compensations, once the intruder's nostrils have adjusted. At the flick of a switch, the lights come on, revealing a warren of chambers, each marked with the number of people it was designed to accommodate. We are standing in one of the bunkers - there were more than a thousand - built in the Nazi era.
The construction is utilitarian: no swastikas or any other artwork of the era adorn the barren concrete walls, only warnings to mind the step, and those no- smoking signs. This particular bunker, approached from the platform of Gesundbrunnen underground station, was built to shelter 1,300 civilians from the bombs.
Once a month a group of 30 people is led down the steps into this twilight world for a two-hour tour of the vaults and the wartime helmets and other artefacts on display. Tourists, especially from abroad, would rather see the bunkers that were once inhabited by Hitler and his henchmen, but those are out of bounds.
Our tour guide, Dietmar Arnold, is saddened by the authorities' lack of interest in the city that lies under their feet. Mr Arnold, an architect who is nuts about bunkers, has worked hard with his non-profit association, Berlin Underworld, to restore two shelters at Gesundbrunnen.
He is regarded as one of the city's foremost authorities on the subject. He has researched wartime archives kept in Moscow and written a book mapping the tunnels the Nazis, and subsequently the Communists and Western intelligence services, dug under the streets. The latter served as conduits for spies or fleeing refugees. He is working on a PhD on Berlin's underground history.
It is the Nazi era that fascinates Mr Arnold most. He has visited most of the bunkers and is convinced many could be reopened as museums. Not the Hitler bunker, though. A concrete chunk left over from the Fuhrer's redoubt was discovered last month by workmen looking for unexploded bombs in the new government districts. The Russians had taken care of the rest - there is nothing left worth showing.
But there are other hide-outs of notorious Nazi leaders that would attract tourists. The former chancellery had two bunkers. One was destroyed but half of the bigger one survives. Mr Arnold has stood in the hideaway of Albert Speer, and adds: "I know where the Goering bunker is. I'd like to open it, but I don't think the government will give permission."
The authorities have not encouraged Berlin Underground's work until now. It is funded by donations, and Mr Arnold confesses he sometimes has to resort to clandestine methods as he charts rediscovered labyrinths.
The government, he believes, would rather leave that part of the city's past in the shadows. "It's a bad history, it's a dirty history, and they'd like to clean up Berlin for the next century." The Goering bunker, near the spot where Germany is building a monument to the victims of the Holocaust, is unlikely to be opened soon.