From around 1850 to about 1941, an illegal practice flourished in Portland, Oregon, whereby able-bodied persons were essentially kidnapped off the streets and sold to sea captains who forced them to work on their ships for no pay. An elaborate network of tunnels below the city provided ample places to hide these captives until they were sold. Trapdoors known as deadfalls were commonly found in the area, and allowed captives to be dropped into the tunnels below the surface where they were held in cells until the ships were ready to sail.

During the Prohibition years, many of the drinking establishments closed above ground, only to re-open in parts of the tunnels. The tunnels run all along the Portland coast and the activities therein caused the Victorian town to become better known as the Forbidden City or the Unheavenly City.

The Cascade Geographic Society offers tours of these rather fascinating tunnels.