Third Infiltration Tunnel
Built by North Korea to initiate a planned (but never executed) surprise attack on Seoul, this tunnel was discovered by South Korean troops in 1978. Two meters high, two meters wide (6.5 by 6.5 feet), and stretching 1,635 meters (5,364 feet) long, the tunnel would have enabled 30,000 North Korean troops to enter South Korea per hour. Visitors can now tour much of the tunnel, which has been preserved as a reminder of the perceived North Korean threat.
