The area north of Tokyo has a high risk of flooding because of a concentration of rivers and being at the sea level. The government’s solution was to build a underground system that would divert excess water from the small rivers, store them in a massive tank until the weather improves, then pump them out to a large river with more capacity for extra water.
Tours of the facility are offered, though only in Japanese. Public transportation is virtually non-existent so you’ll need to catch a taxi or walk 40 minutes from the nearest rail station.
Another video I made explaining this facility from the ground level.
Details on tours