My post about CIncinnati's abandoned subway reminded BB reader Colin Sheridan of Portland's Shanghai tunnels. During the 19th century, this was the real underbelly of the city. Sailors would get drunk, drugged, and dragged through the underground tunnels to the port where they'd be sold to a ship captain as slave labor. By the time the poor saps awoke, they were already at sea. These days, tours are available and, of course, there's even a Shanghai Tunnel bar. Link
Update: BB reader Mike says that Chuck Palahniuk's non-fiction book Fugitives and Refugees: A Walk in Portland, Oregon "covers the tunnels and a bunch of other cool stuff to be found in, around, and under Portland."
Update: BB reader Jeff says "several people have called the tunnel tour operators written about in Fugitives and Refugees, and the conclusion is that the tours are not currently in operation because the building they used to use to enter the tunnels has been renovated and bought by somebody tunnel-unfriendly. They're looking for a new entrance."