Early Seattle was plagued by an inadequate sewage system that caused no end of problems when the tide rose. In the late 19th Century, just as Seattle was becoming a city, it was struck by a fire which destroyed much of the downtown. The City Fathers decided this was a good opportunity to raise the streets, and thus the sewers, to solve the drainage problem once and for all. All well and good, but while they dithered, the merchants started rebuilding, this time with stone and brick. Business couldn't wait.
Faced with buildings which were built to the old street level, the city constructed walls 12-32 feet high at the curb so as to build a sort of trough where the street would go. They could then back-fill these to an appropriate height, install services, and cover them over. To obtain fill for the streets, the city used mining water-cannons to wash down the hill behind the city, and sluice the resultant mud into the brick and concrete trough-streets. This filled the streets and reduced the slope of the hill behind the town from 49 degrees to 18 degrees.
Unfortunately, this procedure also left the store entrances below street level. Undaunted, the stores installed stairs, ladders and even gang-planks from the new street level to their 2nd floors, for access. Gradually, the city proceeded to install sidewalks that matched the new street heights, covering over the old sidewalks as they went. Skylights were installed to illuminate the old sidewalks below. Some are still visible today. Gradually the lower level fell into disuse and the store entrances were moved to the second floor, now ground-level. When plague struck, the city blamed the rats in the underground and sealed it off. It remained that way until 1990.
Underground Tours on Pioneer Square now conducts guided tours of this fascinating piece of Seattle history.
First stop on the tour, a store's former ground floor
Sidewalk skylight from below
Old store windows, now below ground
Barrel-vaulted ceilings on the former sidewalk
Raised commode in early attempt to compensate for flooding problems
More underground storefronts
Left-over elevator parts, abandoned below ground.