The Metropolitan area around Tokyo had suffered flood disasters with the rapidｌy growing population and urbanizations after the second world war. Because of this situation, the “Metropolitan Area Outer Underground Discharge Channel” was established. This flood defense system is really really great. I recommend you to watch this with your own eyes when you have an opportunity to visit the metropolitan area around Tokyo in Japan. You can see like an underground temple shrine sanctuary.
You can see that by the following video:
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