[paulette449]

I went on the Seattle Underground tour, it was really interesting, strongly recommended. The website has more history [1]:

"(Bill) Speidel ultimately did find the remains of the city consumed in the Great Seattle Fire of 1889, a town founded on mostly soggy tideflats whose streets would, whenever the rains came, bloat deep enough with mud to consume dogs and small children. After the fire, which destroyed some 25 square blocks of mostly wooden buildings in the heart of Seattle, it was unanimously decided that all new construction must be of stone or brick masonry. The city also decided to rise up from the muck in which its original streets lay. It was this decision that created the Underground: The city built retaining walls, eight feet or higher, on either side of the old streets, filled in the space between the walls, and paved over the fill to effectively raise the streets, making them one story higher than the old sidewalks that still ran alongside them."

[1] http://www.undergroundtour.com/about/history.html