I have searched for some Seattle underground pictures. That is fascinating.
There was something similar near to me but on a much smaller scale. Due to the building of a bridge to cross a railway, a couple of buildings had their ground floor buried to allow the road to rise. A few years ago the building's wall started to move so they excavated around it and put in a new suspended pavement (sidewalk to our US friends). Whilst the work was going on, it was interesting to see the old ground level and the bricked up doors and windows in the ground floor.
Obviously not on the scale of Seattle but still interesting.