Had the idea, already asked, was inspired by C.H.U.D. and have these to show for it:
https://www.seattletimes.com/seattle-news/seattles-under-cover-world-these-hidden-passageways-might-save-you-some-time/
https://www.yelp.com/search?find_desc=shanghai+tunnel&find_loc=Seattle%2C+WA
https://www.atlasobscura.com/places/portland-s-shanghai-tunnels