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 sneaked 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.
ABOUT THE TUNNELS
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. While some tunnels were 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.
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.
"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.
WHAT YOU'LL SEE
The tours usually start at a labyrinthine Cold War bunker in the bustling immigrant neighborhood of Wedding. Here, the Underworlds Association has built a model tunnel equipped with buckets, shovels and a little wooden 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 to the Bernauer Strasse in the Mitte neighborhood, one of the most popular spots for tunnel diggers at the time, due to the high amount of clay in the soil.
"Today none of the original tunnels (on Bernauer Strasse) are still accessible, but sometimes, during street construction work, unknown ones get discovered," Arnold said.
IF YOU GO ...
Tours of Berlin's escape tunnels
Tour "M" offered in English on Wednesdays at 1 p.m. through the end of October. Tickets are $17.75 (12 euros); $13 youth (9 euros). Other tours offered daily year-round.