The tunnels are hundreds of feet below the earth’s surface and stretch from the North Korean side of the heavily fortified Demilitarized Zone into neighboring South Korea.

One was discovered just 32 miles from the South’s capital, Seoul.

South Korea says the four passages, the so-called Tunnels of Aggression, were built to move thousands of North Korean troops quickly and covertly underneath the Demilitarized Zone and onto South Korean soil for an invasion, an accusation Pyongyang has long denied.

But in the decades since their discovery, some of the tunnels have found new life as a tourist destinations. Thousands of Koreans and foreign visitors explore these odd relics of a frozen conflict, one that is now stressed by renewed tensions and in the spotlight ahead of President Trump’s visit to the Peninsula on Tuesday.