Visitors come to see how desperate East Berliners dug their way to freedom.
When the East German Government built the Berlin Wall in 1961 to prevent its citizens from leaving, the regime failed to account for the ingenuity and creativity of those willing to risk anything to escape the communist system.
Some flew over the barrier in hot air balloons, others sneaked across hidden in secret compartments in cars. And several hundred took advantage of the soft, sandy soil beneath Berlin to tunnel their way beneath the wall.
Today, more than years after the wall's demise, Berlin's Cold War-era bunker and tunnel system has become one of the most popular tourist attractions.
From the 1960s to the 1970s, Hasso Herschel helped dozens escape from the East to the West through the secret tunnels, some of which he dug with his own hands. "This was the best thing I ever did in my whole life," the 74-year-old says.
Herschel regularly escorts groups through the hidden world below Berlin's streets, explaining how the subterranean escape routes worked.
Herschel, who escaped to West Germany with a forged passport in 1961, dug several illegal tunnels underneath the wall, the first in September 1962.
Its entrance was hidden in a house on the eastern side of the border. Twenty-nine people fled through that shaft, making it one of the most successful tunnels.
Some tunnels were only about 30m, others as long as 170m. Some were barely big enough to crawl through, others high enough to stand up in. Altogether about 300 people managed to escape through the tunnels.
Fleeing East Germany was dangerous.
Border guards had orders to shoot escapers on the spot. Researchers estimate that 136 people died trying to cross the wall and about 750 perished along the entire 1400km length of the border separating East and West Germany.
Often tunnels were discovered by the border troops or the Stasi, East Germany's dreaded secret police.
Dietmar Arnold, the head of the Berlin Underworlds Association, which conducts the tours, says: "Most tunnels were dug from the West to the East, often by men who had already fled to the West and who were now trying to get the rest of their family out of East Germany." The tours usually start at a labyrinthine Cold War bunker where there's a model tunnel equipped with buckets, shovels and a little wooden box wagon that was used to carry out the excavated soil. The light in the bunker is dim and fluorescent paint from the Cold War-era glows on the walls, creating an eerie atmosphere.
Later on, the groups move on to Bernauer Strasse in Mitte neighbourhood, one of the most popular spots for tunnellers.
It was then, that people started digging their way to freedom.
"We crawled on all fours through the mud, until we reached a ladder that we climbed up," says Anita Moeller, the sister of Bernauer Strasse. "It took me a while to understand I was free ... and only then I experienced this complete inner happiness."
Getting there: All four Emirates' daily flights from New Zealand provide direct connections at Dubai with the airline's daily services to Hamburg. Fares are from $2582.28 economy class return, all taxes included. Local budget carriers connect to Berlin.
Further information: berliner-unterwelten.de