From 19th century sewers and Nazi-era bunkers to Cold War nuclear fallout shelters, underground Berlin is now breaking surface - and serving as a growing tourist attraction.
"Every time, the numbers we show round are growing," says Michael Foedrowitz, a historian and consultant with the Berliner Unterwelten, a group of historians, archaeologists and urbanists which has been opening up underground Berlin to visitors.
"Many foreigners are fascinated by this too - now we do tours in English, Spanish and German," he adds.
World war horrors
Foedrowitz' group operates from a second world war air raid shelter complex beneath the U Bahn underground station in the north Berlin district of Gesundbrunnen in Germany.
Hidden behind some green steel doors off an ordinary metro stairwell, the shelter once housed thousands of terrified locals, fleeing British and US air raids - and finally Russian troops.
Now, its dark concrete corridors, ventilator rooms, surgeries and guard houses act as a museum to the horror of war 60 years ago.
Gunther Schubbel, a Gesundbrunnen resident who was born in 1939, the year the war broke out, recalls: "I remember one night towards the end of the war in 1945. We were all running down the stairs to the entrance of the shelter after the bombs started falling and one woman fell over.
"Sixty-one years after the end of the war, the authorities reckon some 10,000 people's remains are still to be recovered from beneath Berlin - along with around 75,000 tonnes of unexploded bombs"
Sylvia Brito Morales, a Berliner Unterwelten guide
"When we emerged later, I remember seeing her body, crushed to pieces. Sometimes, when you emerged, everything would be on fire - all the houses would be gone."
Some 1.5 million tonnes of bombs and incendiary devices were dropped on Berlin by the Allies during the war - 800 tonnes a day by 1945 - killing 600,000 people.
Sylvia Brito Morales, a Berliner Unterwelten guide, says: "No one knows for sure how many died.
"Sixty-one years after the end of the war, the authorities reckon some 10,000 people's remains are still to be recovered from beneath Berlin - along with around 75,000 tonnes of unexploded bombs.
"They think it will be 2020 before they've recovered it all."
Cold war fears
A tunnel from Gesundbrunnen also connects to a more recent horror - the nuclear fallout shelter at nearby Blochplatz, built during the Cold War.
A room full of nuclear-biological-chemical warfare suits serves as an eerie reminder of the feared mutual assured destruction of the post-war era.
In those days, the Berlin Wall divided the city in two above ground - while below the East German authorities also tried to divide the troglodyte world as well.
Nuclear, biological and chemical
suits in a nuclear fallout shelter
Dietmar Arnold, the founder of Berliner Unterwelten, says: "They blocked off sewers, communications tunnels and entire metro stops - what became known as the 'Ghost stations'.
"Trains would still pass through under the wall from west to east, but they were not allowed to stop. They would slow, so you could look out and see deserted platforms, left as they were when the wall went up in 1961."
Pulling back a concrete lid, Arnold reveals one of the many spy holes used by the East German police to watch the metro tunnels and make sure they were not being used for any attempted escapes.
Peering into the darkness, a U Bahn train suddenly roars up out of the darkness.
Western and Soviet spy services used the tunnels too, as they attempted to ferry their people in and out of East and West Berlin, as well as tap the other side's telephone lines - while many ordinary East Berliners dug their own tunnels to try and escape to the West.
"Most of these homemade tunnels were so dangerous," Arnold says. "They were not built by professionals and many collapsed. We have uncovered several, but they are too dangerous for us to show people down them now."
"For many years afterwards, none of us who survived ever talked about what it was like down here... We saw some quite terrible things. Now though, we want to say, 'look, this is what happened' because we want people to know what it was like so it can never happen again"
Gunther Schubbel, Gesundbrunnen resident
Given the grim nature of much of this history, Berlin's underground remains a controversial area.
In particular, organisations such as Berliner Unterwelten have drawn much criticism for opening up Nazi era bunkers.
Foedrowitz says: "They've labelled us 'bunker lovers' and some journalists tried to put us in a right-wing corner.
"But we are nothing of the kind. We are serious historians and urban archaeologists."
Then there is the chief underground complex of them all - Adolf Hitler's bunker.
The ruins of this remain buried under a car park close to the very heart of the city, a place marked on many guide books and historical maps of Berlin and usually also marked out by parties of tourists, standing over the site in an otherwise nondescript housing estate.
It has been blown up and filled with rubble, but parts of the giant complex of which it was the centre, are still intact.
One of these sections is the 'Fahrerbunker' - the underground garage where cars for the Nazi elite were parked.
Says Arnold: "I think the Fahrerbunker should also be opened up and made into a museum. Everyone knows that's where it is, and it has taken on this mythological status, with many people coming to see it.
Humboldthain Flak Tower, a Nazi
shelter in Berlin
"So let's demystify it - make it into a place where the horror can be explained."
But Berlin's city authorities disagree.
Manuela Damainakis, spokeswoman for the Berlin City Senate's Urban Development Department, told Aljazeera.net: "We had a discussion about the bunker.
"A decision was made. There is no future for it - there will be no signs for it, and everything will be built up around it."
A big factor behind the decision to leave this bit of the past well and truly buried was a concern the bunker could become a shrine for neo-Nazis.
The Urban Development Department's Damainakis says: "Can you imagine if there was the possibility for having a place where such people would gather right in the centre of Berlin?
"We don't want that. We don't want to give people places to remember these things, to commemorate them."
Yet the bunker's 8-foot thick cement roof and massive walls have proved tough to crack, with several attempts to demolish the structure leaving uncertain results.
How much remains under the car park off Voss Strasse seems largely unknown.
The Cold War too provokes controversy, with discussion over how the period - and the division of the city in particular - should be remembered.
In 2004, an unofficial memorial consisting of 1065 crucifixes to those who died trying to cross the divided city was set up at the notorious Cold War Checkpoint Charlie.
After furious discussion in the Senate, in July 2005, the tribute was pulled down.
"Can you imagine if there was the possibility for having a place where such people would gather right in the centre of Berlin? We don't want that. We don't want to give people places to remember these things, to commemorate them"
Manuela Damainakis, Berlin City Senate's Urban Development Department spokeswoman
Now, the Senate intends a new memorial instead.
"We're talking about something more than a museum," Damainakis says. "We thought it would not be the best idea to put a memorial just in one place, as the wall was everywhere, a part of the whole city.
"So what we're thinking is of something where people will be shown around Berlin to places where the division was most evident. We don't just want a 10-minute tour."
Berlin has already produced some striking monuments to commemorate the darker pages of its history - and of the history of all Germany too - with the Jewish museum, the Holocaust monument and the museum to the East German secret police, the Stassi.
Yet in some ways, the chill of the underground city, in its stark concrete walls and echoing chambers, remains one of the most thought provoking memorials of past horror.
"For many years afterwards, none of us who survived ever talked about what it was like down here," says Gesundbrunnen resident Schubbel.
"Not even to each other. And nobody ever asked us either. Now, most of us are gone. I was a little boy back then. We saw some quite terrible things.
"Now though, we want to say, 'look, this is what happened' because we want people to know what it was like so it can never happen again.
"That's what it's really all about - stopping it from ever happening again."