Seattle's underground tour is half comedy routine and half history lesson. The story of how the underground came to be and the way the tourguides tell it is a lot of the reason to see the underground tour. So as not to spoil the tour if you ever plan on going, I will give a very short capsule version. Seattlites a long time ago made a lot of stupid decisions. They decided to raise the level of the town (above the mud) and the city and townspeople had a slight disagreement about it. The city raised the streets to ten feet above sidewalk level, and chaos ensued. Eventuall, the sidewalk was raised and the second level of the buildings became street level. The old first levels are still there, underground.
I have found a lot of references to the Seattle Underground being haunted, but the only source I could find is the book Ghost Stories of Wasshington by Barbara Smith. In it there are reports from a tour guide named Janet having seen a man in period clothing. There are a few other references to sightings in the book, and a more detailed description of this one if you want to check that out for further reading.
Cannot really say much more about it than I have already in the about section, except to say that it was fascinating. I loved the lecture/stand-up routine that starts off the tour, and I loved the tour itself. I highly recommend seeing this if you are ever in the Seattle area.