The Kanmon Straits or the Straits of Shimonoseki is the stretch of water separating Honshu and Kyushu, two of Japan's four main islands. On the Honshu side of the strait is Shimonoseki and on the Kyushu side is Kitakyushu, whose former city and present ward, Moji, gave the strait its "mon". The straits silt up at the rate of about 15 centimetres per annum, and dredging has made it possible to build the Kitakyushu Airport at low cost.