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...