My understanding of the Chicago case is that most of what the old "ground level" was is just sewers today. I heard some of it was railroad tracks and maybe some are still used for cargo-only trains, but most of Chicago wasn't really raised a full "floor" with enough headroom for interesting underground spaces.

A US city often overlooked for some intricate people explorable underground spaces is Cincinnati: https://www.visitcincy.com/blog/post/unmistakably-cincinnati...

Some of Cincinnati's underground exists from plans to build subway trains that never completed. I think that makes Cincinnati's particularly sad being that it constitutes a perpetually unfinished public works/public transportation project.

Relatedly to that, Atlanta also has a tiny underground leftover from passenger train lines that ended passenger travel decades ago (and so was turned into a mall, because America): https://en.wikipedia.org/wiki/Underground_Atlanta