Tokyo Metropolitan Area Outer Underground Discharge Channel

12 hours ago (japan.travel)

Weren't tours restricted some time ago?

You can go down, you can look at the place from one spot, bit you no longer can walk around.

This is one of the best things anyone interested in one of the most impressive engineering marvels ever can go and visit anywhere in the world. I cannot stress enough how good it is and how involved the tour gets (wearing waders to go waist deep in water to view the jet turbines used to pump water).

I strongly advise taking someone who can help translate Japanese with you as there's a huge amount of information to take in and it's a fantastic opportunity for anyone keen on photography as well.