To wit, the G-Cans Project.
I first learned of this through a post on Boing Boing, and later did some research on my own to make sure it was authentic. A summary:
"The G-Cans Project is a massive project, begun 12 years ago, to build infrastructure for preventing overflow of the major rivers and waterways spidering the city (A serious problem for Tokyo during rainy-season and typhoon season). The underground waterway is the largest in the world and sports five 32m diameter, 65m deep concrete containment silos which are connected by 64 kilometers of tunnel sitting 50 meters beneath the surface. The whole system is powered by 14,000 horsepower turbines which can pump 200 tons of water a second into the large outlying Edogawa river."
Have a look at the photo gallery if you have the chance. I look at this and feel like I'm inside an Anime film. And all the time, I'm hearing an echo from the voice of Sidney J. Mussburger at Hudsucker Industries: The Future is Now.