Tokyo is protected against flood by gigantic constructions under the ground. They are invisible in our daily life, but they sure help us to remain safe when Tokyo is hit by torrential rain. One such example is the Kanda River Underground Retention Basin made up of 4.5 km tunnel 40 meters under the ground. This underground retention basin can hold up to 540,000 tons of water when Tokyo is hit by flood. Another example is the Metropolitan Area Outer Underground Discharge Channel. This is a complex of five vertical shafts of 30 meters in diameter and 70 meters in depth, which are connected each other by tunnels of 10 meters in diameter and 6.3 kilo meters long, all told. This is actually located in Saitama prefecture, but it was constructed to protect broad area of Kanto including Tokyo. Each vertical shaft is as huge as space shuttle. At the end of the channel, there is the symbolic place called ‘pressure-adjusting tank’ also known as ‘Underground Parthenon’ which is as large as a soccer ground and measures 18 meters in height. These facilities accept a study tour. It might be a good idea to visit and see how they work, when you have a time. ( By Hiroshi Ayukawa from www.quora.com )
