The one that made me think of this is Metropolitan Area Outer Underground Discharge Channel in Kasukabe, but I’m sure there are others. If you find a list of civil engineering tours available, please share!

Tokyo Metropolitan Area Outer Underground Discharge Channe - https://news.ycombinator.com/item?id=43816183 - April 2025 (6 comments)

Tokyo's Underground Discharge Channel - https://news.ycombinator.com/item?id=19785044 - May 2019 (40 comments)

Tokyo Is Preparing for Floods ‘Beyond Anything We’ve Seen’ - https://news.ycombinator.com/item?id=15436943 - October 2017 (210 comments)