Ok, its underground, but its not really a separate society or hidden or anything. Well, it is hidden, as its underground and not everyone can get to it.
Anyways, what it is, is that the downtown area long ago used to be all wood buildings. There was a big fire that burned everything down. SO they decided to build everything up in stone. However, before Seattle used to be flats with a cliff behind it. But they decided to level/slope this grade after they built those buildings, so more or less, they built up retaining walls around the first (to sometimes third, 8-30 ft) floors of those buildings, filled in the streets with crap and then lived like that for awhile. Then they realized how stupid it was to have sidewalks below the streets, so instead of filling in the sidewalks (which would make the buildings less stable), they just covered up all the sidewalks. So now almost every old building's first floors are underground, and not used. I hope that explains it.