G-Cans Saitama, Japan. It's the Metropolitan Area Outer Underground Discharge Tunnel, designed to provide flood control for Tokyo, basically. One of the largest (if not the largest) underground structures in the world. Excess rainwater runoff is collected and then pumped out of this reservoir at a rate that the surrounding rivers can accommodate, preventing flooding. It's an engineering marvel.
http://www.ktr.mlit.go.jp/edogawa/works/saigai/sonae/gaikaku/frame_index.html
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