In October 1964 Tunnel 57 under the Berlin Wall  was the location of a mass escape by 57 East Berlin citizens to West Berlin. Built from the basement of an empty bakery in West Berlin, the tunnel ended in a disused outhouse in East Berlin.

At a depth of 12 meters (40′), and a length of 145 meters (475′), Tunnel 57 was the longest, deepest and most expensive flight tunnel built in Berlin.

Source: Wikipedia