Topsoilers are the residents of Topsoil, or the surface world. Most of the Topsoil population includes humans and animals. They live on all of the seven continents and have different ways of living. Topsoilers were largely unaware of the underground universe in which the Tunnels series takes place, but often the Styx's plots involve destruction of Topsoilers and their world. Notable Topsoilers include Will Burrows (who was actually born a Colonist), Chester Rawls, Dr. Burrows, and Drake.
It is unknown how humans came to live Topsoil. They were reared to serve the Styx in the Garden of the Second Sun (inner world), but somehow escaped to the surface. It is possible that humans lived Topsoil during the same period (Terminal states that the Styx "went Topsoil to set things right..."), but it isn't stated directly. Some Topsoilers, such as Gabriel Martineau, have had the instinct to return to the underground and built/lived in The Colony. The Styx have released plagues and had Phases throughout history, but the Topsoil world still survived.