Super interesting. Essentially, they built an underground river which is connected to many of Tokyo's rivers (many kilometer underground tunnel) that diverts overflow water from the rivers to prevent catastrophic flooding.

It cost 3 billion dollars and took 13 years to build.

I would like to see how these held up during the tsunami.

Japan has spent a LOT of effort to mitigate floods. It does mean that all river banks anywhere near a city look the same, and very artificial.