When this tunnel was discovered in October 1978, North Korea insisted that South Korea dug it to aid an invasion of the north. Of course, that turned out to be untrue -- the North Koreans dug the tunnel themselves. The underground tube is 1.6km (about a mile) long, 1.95m (6 1/3 ft.) high, and 2m (6 1/2 ft.) wide. It penetrates 435m (1,427 ft.) south of the demarcation line at a point only 4km (2 1/2 miles) south of Panmunjeom (and is the closest to Seoul). It runs through bedrock about 73m (240 ft.) below ground. Almost identical to the Second Infiltration Tunnel, it enables about 10,000 armed soldiers to move through it in an hour. To enter the tunnel without a tour, you must buy a ticket at the Imjingak DMZ ticket office. The ticket includes a shuttle to the Dorasan Observation Platform and the Dorasan station (and sometimes Unification Village). Soldiers will guide you, but they only speak Korean (so it's best to arrange an English-language tour before you arrive). There is a monorail that takes you halfway into the tunnel (walking down the entrance is very steep and narrow). The tunnel may be a bit cramped for you if you are tall (although they do provide helmets) and you should avoid it if you're claustrophobic or have heart problems. No photos are allowed in the tunnel.