I’ve been to Chicago many times. I always thought that the sections under Michigan Ave, like where the Billy Goat Tavern is IIRC, were just built that way.
Yeah the billy goat is actually at ground level as is lower Michigan and lower wacker dr. A lot of the older buildings actually have 3 basements. 2 which is are above ground level and one dug in the earth. I worked in the Harris bank building and the vaults were down in the 2nd and third basements.