神田川
Construction began in the 1980s of a diversion tunnel which could temporarily store excess water which historically has been a big problem causing flooding to thousands of homes during powerful typhoons or torrential downpours. The Kanda River Underground Diversion Channel is actually one huge tunnel built under major roads. The plan is to construct a 4.5-km tunnel, 13 m wide tunnel 40 meters below street level. The first 2 km section was completed in 1997, and it can hold the runoff of 50 mm of rain falling in one hour. The reservoir has drawn water from the river on more than a dozen occasions since it opened, and the Kanda River has not flooded during that time. Plans call for the tunnel to be extended to Tokyo Bay, literally creating a second, underground Kanda River.