The city of Seattle has a long history of terrible urban planning.
This tradition began with the earliest settlers, when they decided to build their settlement on a tidal marsh. Streets were filled in with loose dirt, which would frequently subside. Potholes became so big that they were listed on the maps of the day, and due to the backpressure of the incoming tide, ground-floor toilets would explode when flushed. Three to four foot geysers of raw sewage were reported. Finally, a young boy drowned attempting to cross what is now 1st Avenue South, and the city decided something had to be done. The level of the entire downtown area would be raised between 8 and 35 feet, with the current street level becoming basements and upper floors becoming the new street level entrances.
Today you can book a tour down below the sidewalks and see some of the old business fronts rotting away beneath. Immortalized in popular imagination, the main appeal of the Underground Tour is really the fascinating and hilarious historical lectures by the tour guides, who are all professional comedians. Visually, the Underground City looks pretty much like a bunch of disused old basements. (Sorry, "Kolchak, The Night Stalker" fans, the movie wasn't filmed in the real "underground city", it was a much more impressive-looking Hollywood soundstange.) Here's my photos.
For a good review of the Underground Tour including some comedic highlighs, see http://weeklywire.com/ww/08-23-99/memphis_trvl.html.