This cavernous complex that has been called Japan's underground "Parthenon" is charged with protecting Tokyo and surrounding areas from catastrophic flooding.
Above ground, there is little to give away the cathedral-like feat of engineering that forms the main reservoir of the Kasukabe flood tank, the largest facility of its kind in the world.
The immense structure - deep enough in some parts to hold the Statue of Liberty in New York City - 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.
Staff at the facility in Saitama, north of Tokyo, are on constant alert, especially during Japan's rainy and typhoon seasons from June to late October.
But experts warn that more of such flood-protection structures may be needed, as global warming makes what were previously once-a-century storms increasingly common, and catastrophic.