The Seikan Tunnel is a railway tunnel in Japan that connects the islands of Honshu and Hokkaido. Completed in 1988, it is one of the longest undersea tunnels in the world, measuring about 53.85 kilometers (33.5 miles) in length. The tunnel was built to provide a reliable transportation route, especially during harsh winter weather when ferries were often disrupted. The construction of the Seikan Tunnel took over 17 years and involved significant engineering challenges due to the region's geology and seismic activity. It was designed to withstand earthquakes and tsunamis, ensuring safety for passengers traveling on the Hokkaido Shinkansen and other trains that use the tunnel.