From the bridge Troy explained to us that Chicago was built on what was once swamp land. This lead to problems (mostly sewage) as the population expanded. The engineers had to sink steel beams deep into the swamp and raise a foundation up above the swamp. They brought in earth to fill in the space that had been created between the streets of the city and the swampy land beneath. While the city was being lifted, in the dark, dank crevices, derelicts, gangsters and the like, made their homes under the city streets and speakeasies flourished. This, said Troy, was the origin of the term “underground.” These shady types were part of the underground.