[acyou]

22 meters underground, built in 1950s, Tokyo, 5-7% of GDP - yeah the gigantic underground vaults serve as flood protection, to those who have a good understanding of Japanese history, it's understandable to believe these were rather primarily built as bomb and nuclear shelters.

[hollerith]

It would easy to tell: (unless they were occupied by only a handful of people) they would be useless as fallout shelters without a lot of ventilation, and you can't rely on the wind: you'd need big fans and ducts, and the fans would need to keep running after most of the electricity-generating capacity available in Tokyo is destroyed, i.e., this ventilation system would need its own hardened electricity supply.

[bobthepanda]

Also, using this as a shelter would have a pretty obvious downside if torrential rainstorms happened.