For the most part, every inch of the world can be explored. If you can fly, drive or walk to it, it’s fair game. The Demilitarized Zone between North and South Korea represents one of the few stretches of land that is off limits, or at least the surface of it is. There are tunnels, it turns out, underground that can be visited.

Called the Third Tunnel of Aggression, it was built by North Korea in the 1970s and discovered in 1978 by South Korea. It was the third tunnel found by the South—a total of four have been discovered—and it is rumored there may be more than a dozen more undiscovered North Korean passages. At the time of its discovery, the United Nations estimated the North could move 30,000 men per hour through the tunnel to the Southern side.

After initially denying its existence, the North claimed that the Third Tunnel was a coal mine, going as far as to rub black coal dust on the walls. That tactic obviously failed and the South took control of the tunnel, blocking off the actual demarcation line with concrete barriers.

Nowadays, the passage snakes beneath the divided nations for over a mile and, at its outset, looks like a space-age tunnel from a science fiction movie. Although the tunnel is available for visitors on tours and even begins with a gift shop, deeper down the two-by-two-meter tunnel are stark reminders of the danger the tunnel represents.

As you walk along the path, the light dims and visitors are left only with moist, craggy walls and the faint flickering lights installed along the way. You can walk a full 265 meters until hitting the divider installed to protect the South from attacks by the North.