Seikan Tunnel, an undersea railway tunnel between the Japanese islands of Honshu and Hokkaido. With a length of 33.5 miles (53.9 km), it is the world's longest tunnel. It contains two tracks, allowing trains to run in both directions through the tunnel at the same time. Seikan Tunnel was completed in 1988 after 24 years of construction. It was bored largely through volcanic rock and reaches a depth of 790 feet (240 m) below sea level, some 325 feet (100 m) below the seabed of the Tsugaru Strait.