The Metropolitan Area Outer Underground Discharge Channel (首都圏外郭放水路 shutoken gaikaku hōsuiro?), is an underground water infrastructure project in Kasukabe, Saitama, Japan. It is the world’s largest underground flood water diversion facility, built to mitigate overflowing of the city’s major waterways and rivers during rain and typhoon seasons. It is located between Showa in Tokyo and Kasukabe in Saitama prefecture, on the outskirts of the city of Tokyo in the Greater Tokyo Area, Japan.
Work on the project started in 1992 and was completed by early 2006. It consists of five concrete containment silos with heights of 65 m and diameters of 32 m, connected by 6.4 km of tunnels, 50 m beneath the surface, as well as a large water tank with a height of 25.4 m, with a length of 177m, with a width of 78m, and with 59 massive pillars connected to 78 10 MW (13410.22 horsepower) pumps that can pump up to 200 tons of water (the approximate equivalent of a standard 25 meter pool) into the Edo River per second.
“Ryukyukan” for Underground Exploration Museum of The Metropolitan Area Outer Underground Discharge Channel is also a tourist attraction and can be visited for free; however, as the tours are conducted in Japanese, a Japanese speaker must be present in the group to act as a translator for non-Japanese speakers.