we took bill spiedel's "underground" tour. here's the story: seattle was built on mud flats, at the base of a big cliff. the tide would come in, and the roads would get all mucky & gross. the sewage system would go backwards all the time. at some point, there was a huge fire that burned down most buildings. half the people said they would start rebuilding immediately, half said they wanted to wait until the cliff was blown away so that buildings could be built well above the water level. for awhile there were walls at the edges of streets with ladders leading up to the new sidewalk levels. just incredible to picture. anyway, after making the new roads & sidewalks, the old ones still existed. they put in the skylights (which used to be clear, but turned purple with age due to the high amount of manganese in the glass) to let in light so people could still shop & see where they were going. for awhile businesses kept storefronts in the new "underground," but as one might guess, business didn't do so well & the usage died out. the underground was eventually condemned. here are pictures of glass skylights in the sidewalks from below the sidewalks and above.