East Germans successfully defected by a variety of methods: digging long tunnels under the Wall, waiting for favorable winds and taking a hot air balloon, sliding along aerial wires, flying ultralights and, in one instance, simply driving a sports car at full speed through the basic, initial fortifications. When a metal beam was placed at checkpoints to prevent this kind of defection, up to four people (two in the front seats and possibly two in the boot) drove under the bar in a sports car that had been modified to allow the roof and windscreen to come away when it made contact with the beam. They lay flat and kept driving forward. The East Germans then built zig-zagging roads at checkpoints. The sewer system predated the Wall, and some people escaped through the sewers, in a number of cases with assistance from the Unternehmen Reisebüro. In September 1962, 29 people escaped through a tunnel to the west. At least 70 tunnels were dug under the wall; only 19 were successful in allowing fugitives—about 400 persons—to escape. The East Germany authorities eventually used seismographic and acoustic equipment to detect the practice. In 1962, they planned an attempt to use explosives to destroy one tunnel, but this was not carried out as it was apparently sabotaged by a member of the Stasi.