It has been called Japan's underground "Parthenon," a cavernous complex charged with protecting Tokyo from catastrophic flooding – a risk experts warn is growing as the climate changes. Deep enough in some parts to hold the Statue of Liberty, the immense structure funnels away and redirects excess water from storms and typhoons, protecting one of the globe's most populous capitals.

Soaring pillars weighing 500 tonnes each support the main reservoir, a bare concrete tank the length of two football fields. In western Japan's Osaka, a flood facility similar to the Kasukabe reservoir is being built at a cost of 366 billion yen ($3.5 billion). Construction is scheduled to finish in 2044.