For years, people living by the Tonegawa, Arekawa, and Edogawa Rivers faced the threat of terrible floods during the typhoon season. Every few years, anywhere between a few dozen and tens of thousands of houses would be inundated with water, and as the Tokyo metropolitan area expanded, the problem was only growing worse.
In the early 1990s, someone came up with the idea of huge underground discharge tunnels for the rivers to prevent flooding. Construction started in 1993, and 13 years later, the G-Cans Water Discharge Tunnel was completed. It’s a really impressive structure, and and the facility is open to tourists. I visited this December and found it to be really impressive.
There are a bunch of videos here. The website and videos are in Japanese, but just click on “broadband” or “narrowband” to watch them.
Official Site (mainly in Japanese): http://www.ktr.mlit.go.jp/edogawa/project/g-cans/frame_index.html