It is believed the tunnels were built to allow a North Korean military invasion. Each tunnel is big enough, so an entire infantry division can pass in one hour, but the tunnels are too small for tanks or other vehicles. All the tunnels run in a north-south direction and do not have branches. Engineering within the tunnels, following each discovery, has become progressively more advanced. For example, the third tunnel sloped slightly upwards as it progressed southward, to prevent water stagnation. Today, visitors may visit the second, third and fourth tunnels through guided tours.