Styx Mines originally started as exactly what it sounds like - a mining operation. It was started as a replacement for Ferra's mine after it was hit by plague, but as more and more space was cleared underground and more and more things such as food, housing, and clothes were needed for workers, it evolved into the underground sprawling city it is today.
Under the surface (haha pun), there is a lot more going on and it isn't the "haven for honest workers" that it touts itself as. The CEO of Styx Mines, Kovus, is a cruel and cynical old man who believes he owns every person ever born on Styx. People are expected to begin work at age 12 and are expected to continue until death. Work conditions in and of themselves are dangerous, and many workers do not live past 35.