When my husband and I told people we were visiting Seattle, one thing that kept coming up as something to do was an underground tour. We did our research, and found Beneath the Streets.
What drew us to this tour was that it was historical rather than comical (apparently another tour is based on and advertises as funny/adult humor). We did the Underground History Tour and had about 10 of us in the group. I appreciated that this tour company keeps the groups small.
Our tour guide was very informative and we learned a lot about the history of Seattle and how the "underground" came to be. I quote the word underground because, like our tour guide said, it is technically not underground - this was the original Seattle that was literally built on top of to what we see today. He made the tour interesting and appreciated how he did it more "off the cuff" than just reading from a script. Also he had a good knowledge of the area, so got some advice for places to eat and things to see (like The Central bar where Seattle grunge came to be) as well.
I would recommend this tour for anyone looking to learn about a not well known part of Seattle history, and be able to step inside it too.