Tokyo flood reservoir

KASUKABE, Japan — The cavernous underground cisterns here north of Tokyo could hold the Statue of Liberty, a scale that underscores the site’s immense task: protecting the world’s most populous metropolis from flooding.

Linked by tunnels that divert water away from the region’s most vulnerable floodplains, the $2 billion underground anti-flood system, completed in 2006, is an extraordinary example of the defenses that global cities are readying as they face an era of extreme weather brought on by climate change.