Entrance to the Seattle Underground Tour in Pioneer Square http://pnwtravels.hubpages.com/hub/seattle-underground-tour
The Underground tours!!! So cool and spooky. Not sure about this particular company, but in general totally worth doing. Underground opium dens, prostitution, speakeasies and shanghaiing (see : http://en.wikipedia.org/wiki/Shanghaiing).
18unusual things to do in Seattle
Underground Tour. Seattle, WA. An old city lies 8 feet beneath the modern one, thanks in part to the Great Seattle Fire of 1889.
The Seattle Underground tour is one of the most interesting things to do in Seattle. Most of the tour takes place beneath the streets and sidewalks...
Insider Travel Tips - Things to see and do in Seattle, Washington
Seattle underground tour - Most would never realize we have a city upon a city... squares on top of picture has people walking on it!
Mercat Ghostly Underground Tour: Edinburgh Vaults. Things to do in Edinburgh, Scotland #review
There's so much history in Rome that lies beneath the surface! Go underground on a tour of the Crypts and Catacombs. Read more: http://justinpluslauren.com/underground-tour-of-rome/