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 20s 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.