I didn't get to do near as much as I wanted in the Seattle area, but I did get to go on the Underground Tour in the summer of 2010. It's well worth it.
A good portion of Olde Seattle was originally much lower on the marshy flatland just off Puget Sound.
Kinda hard to see the photo, but the present day street goes right over the tops of the buildings and the space between it. Those are the buildings which are part of this tour.
Looking up at a skylight. The clear glass was treated with manganese and that has turned the glass to rich shades of purple over time.
It should be noted that some people with claustrophobia would not like this tour at all. It includes navigating through narrow doorways, stairs and passages, with fairly low ceilings. There are three sections of this tour, and so it is broken up with trips up to the streets and sidewalks above for a few feet before descending again to the rooms below.
Bill Speidel's Underground Tour is located in Pioneer Square. Seattle is a very difficult city in which to navigate by car, I'm not gonna lie. At least it was for me. Find a parking garage near downtown, grab a map and just walk from there.