Behold the G-cans project.
Since this entire state around tokyo is actually in a valley it's really prone to flooding with two major rivers running either side, lots of typhoons and lots of rain. Since so many people live here Japan govt decided to build the worlds largest underground river, to basically divert the water underneath the city and back out to the ocean.
Ever seen Lord of the Rings? When gandalf takes them to the mines of moria for the first time and there's a big open chamber sprawling off into the distance? It was kinda like that albeit more realistic.
