Watch Document 72 Hours Season 9 Episode 23 Online

It is 780 meters long, dead straight and has few distinguishing features, but the undersea pedestrian tunnel running 60 meters underneath the Kanmon Straits in southwestern Japan attracts a steady stream of visitors. Many people use the tunnel to travel between Fukuoka Pref. and Yamaguchi Pref., but others have different purposes in mind. The visitors are diverse: from a woman in her 20's who brought her boyfriend to the tunnel that is filled with childhood memories, a man who comes to secretly train for a marathon, to a woman who walks there every week with old friends no matter how busy she is. What draws them to this nondescript tunnel that remains unchanged, day after day? We spend 3 days looking at who walks through this underground passage.