The Awakening, and Mt Rainer going kablooey, plus some creative liberties with the geology under Seattle all helped as well. As is always the case, certain creative liberties get taken to serve the greater good
http://metro.kingcounty.gov/tops/tunnel/tunnel-map.htmlOn that map there, most of the area for about 10 blocks or so around Pioneer Square is what was buried. Those formed the initial basis of the Underground. With the help of a group of dwarves, the original orks who fled to the Underground built up and reinforced that area, then began tunneling and building outward, connecting up with other natural underground formations and structures that were similarly buried.
The Tourist Highway is the "public" section of the Ork Underground, the part that regular, non-underground folk (Topsiders, as they're referred to) can pay to tour, was built along most of the by-2050 abandoned Transit Tunnel that you can see in the linked map. The two most well known public entrances, the Big Rhino (1st & Senaca) and Lordstrungs (5th and Pine). Conveniently fall along a portion of that TRansit tunnel.
The southern end of the Transit tunnel also conveniently sits on the South-Eastern edge of the Renraku Arcology (Matching up the old 1st ed Downtown street map to this), and of course there's been a long rumored entrance to the Underground under the Arc.
