David took me on the underground tour in Seattle. It was amazing, the city's history is actually pretty cool. If you don't know anything about it you should definitely look it up, it was very interesting.
In short the original Seattle streets were too low causing a serious sewage problem. One day there was a huge fire and they had to rebuild. They saw this as a great opportunity to fix the nasty sewage problem. The city started to rebuild the streets way above the original streets, however it was going to take awhile to rebuild the new streets. So the businesses decided remained open during the construction. So you had the new streets above being built and the original street's below where you also had the store fronts. So people would climb up and down to go in and out of stores. Which leaves us today with the underground.