world's first under-ocean tunnel (tunnels and underground excavations), stretching 2.25 miles (about 3.5 km) between the islands of Honshu and Kyushu in Japan. During its construction (1936–44), nearly every known tunneling technique was employed: rock tunneling by drilling and blasting, shield tunneling in soft ground, tunneling with compressed air, and the immersed-tube method involving the sinking of pneumatic caissons. In the 1950s a highway and pedestrian tunnel was constructed east of the original rail tunnel; following a slightly shorter route and using improved equipment, it was completed with little difficulty.