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Construction of the underground Furukawa reservoir continues on the Furukawa river in Tokyo, Japan, on Thursday, Aug. 7, 2014. The underground regulating reservoir tunnel is 3.3km long and its bore is 7.5m. When it's completed in 2016, the 3.3-kilometer (2-mile) reservoir will be able to handle 135,000 cubic meters of water, enough to fill 54 Olympic-sized swimming pools. Photographer: Noriyuki Aida/Bloomberg via Getty Images

Construction of the underground Furukawa reservoir continues on the Furukawa river in Tokyo, Japan, on Thursday, Aug. 7, 2014. The underground regulating reservoir tunnel is 3.3km long and its bore is 7.5m. When it's completed in 2016, the 3.3-kilometer (2-mile) reservoir will be able to handle 135,000 cubic meters of water, enough to fill 54 Olympic-sized swimming pools. Photographer: Noriyuki Aida/Bloomberg via Getty Images