during world war ii, yokosuka was bombed on april 18, 1942 by american b-25 bombers in the doolittle raid with little damage as a retaliation to the attack on pearl harbor. aside from minor sporadic tactical air raids by united states navy aircraft, it was not bombed again during the war; however, from 1938 to 1945 more than 260 caves in more than 20 separate tunnel/cave networks were built throughout the area, with at least 27 kilometers of known tunnels within the grounds of yokosuka naval base. many more tunnels are scattered throughout the surrounding areas. during the war, these tunnels and caves provided areas in which work could be done in secrecy, safe from air attacks. a 500-bed hospital, a large electrical power generating facility, and a midget submarine factory and warehouse were among the many facilities built. american occupation forces landed at yokosuka on august 30, 1945, after the surrender of japan, and the naval base has been used by the us navy since that time. the caves were used for storage and as an emergency shelter during the korean war.