Tokyo's Vast Underground Temple-Drains: The G-Cans

image credit: Dddeco cc
The G-Cans Underground Temple in Saitama is probably the most massive underground flood management system in the world, comprised of miles of tunnels connecting 5 vast silos and one immense water tank: The Temple. The complex spans between Showa in Tokyo Kasukabe in Saitama, with the power to pump 200 tons of water per second into the Edogawa river.
(thanks Stanley)