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.
While some flew over the barrier in hot air balloons, others sailed far around it across the Baltic Sea and still others snuck across, hidden in secret compartments in cars.
But several hundred took advantage of the soft, sandy soil beneath Berlin to tunnel their way beneath the wall.
Today, almost 20 years after the wall's demise, Berlin's Cold War-era bunker and tunnel system has become one of the most popular attractions for tourists and locals alike.
In 2008, more than 150,000 visitors explored the underbelly of the German capital, touring through the deserted bunkers and tunnels that serve as yet another spine-chilling reminder of the city's tense and violent role in the 20th century.
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 retiree said recently.
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, right across from the wall on the Bernauer Strasse, according to Herschel's sister Anita Moeller, who helped him to cross.
"We went into the house, down to the basement, and then had to get into a hole in the floor," said Moeller, who escaped with her infant daughter and husband. "First I was worried, because I'm claustrophobic. I'm afraid of dark and narrow places ... but once I was inside the tunnel, there was no time left for my fears."
Twenty-nine people fled through that shaft, making it one of the most successful tunnel projects at the time.
While some tunnels were just less than 100 feet long, others were up to 557 feet in length.
Some were like small tubes, barely big enough to crawl through, while others were tall enough to stand up in. It took between three days and six months to dig the various constructions between October 1961 and April 1982. Altogether about 300 people managed to escape through the tunnels.
Fleeing East Germany was dangerous. Border guards had orders to shoot any escapees on the spot. Researchers estimate that 136 people died trying to cross the wall and about 700-800 perished along the entire 856-mile length of the border separating East and West Germany.
It is not clear how many were killed trying to flee through the tunnel system. Last month, the city honored Siegfried Noffke and Dieter Hoetger, who were caught by East German authorities on June 12, 1962, while digging a tunnel. Noffke was killed and Hoetger survived but was badly injured.
Often tunnels were discovered by the border troops or the Stasi, East Germany's dreaded secret police, before they could be used. Others collapsed accidentally, were flooded by ground water or buried by loose soil.
"Altogether we have counted 71 tunnel projects and 20 percent of those were successful," said Dietmar Arnold, the head of the Berlin Underworlds Association, which conducts the tours and works on opening more subterranean structures to the public.
"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," Arnold told visitors during a recent tour of the tunnels.
The tours usually start at a labyrinthine Cold War bunker in the bustling immigrant neighborhood of Wedding. Here, the Underworlds Association has created an illustrative 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 neighborhood, one of the most popular spots for tunnel diggers at the time, due the high amount of clay in the soil.
At least 15 attempts were made at Bernauer Strasse to dig a path to freedom through the soil, according to Arnold.
"Today none of the original tunnels are still accessible, but sometimes, during street construction work, unknown ones get discovered," Arnold said.
In the first months after the erection of the Berlin Wall on Aug. 13, 1961, about 600 refugees ran away through the city's canals and the subway system, but by the end of 1961, East German border troops had sealed off access completely.
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," Anita Moeller remembered. "It took me a while to understand I was free ... and only then I experienced this complete inner happiness."