Deep in the Tokyo suburbs, behind a grey door in an empty field, and down a long narrow staircase, is a 177m by 78m cavern. Its 18m-high ceiling is supported by fifty-nine 500 ton pillars. Designed to help channel rainwater safely into the Edo river and help protect the flood-prone Naka basin, the cavern has been dubbed Saitama Prefecture’s Parthenon.
The project’s official name – the Water Discharge Tunnel on The Outskirts of the Metropolitan Area – is less charismatic. But the facility is a breathtaking feat of Japanese civil engineering. As well as the cavern, it includes five underground water storage tanks joined by 6.3km of tunnel, each large enough to house the Statue of Liberty. The system’s four pumps are driven by modified aircraft turbine engines with enough power to drain a 25m swimming pool every second.
Since its completion in 2002 the system has been used more than 60 times, saving countless residents from injury and thousands of homes from destruction.