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Environment / Climate change

Tokyo is sitting on the world's biggest flood run-off tunnel

If nature had its way, Tokyo would be very soggy indeed. The region’s rivers once flowed over the agricultural landscape, swelling or receding according to the volume of water in the system. But when your city looks like this, unpredictable overflow isn’t ideal:  

So to prevent the rivers spreading onto all that urbanisation, the Tokyo government decided to invest in some fairly major infrastructure. In the 1990s, it began construction of a network of enormous underground reservoirs and tunnels which would suck up excess water from four nearby rivers and redirect it into the Edo river. The system was completed in 2009.


The image at the top of this article shows the huge underground water tank, 22m below ground level, which adjusts the water’s pressure. Its official name is the Metropolitan Area Outer Underground Discharge Channel, and it’s the world’s largest underground flood diversion facility. 

Despite its size and cost, the flood diversion system is only used about seven times a year, and before you ask, yes, you can take a guided tour through it whenever it’s not in use. Much more of a draw than Tokyo’s 2020 Olympics, if you ask us. 
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