Humans are kind of gross. We collectively create a constant stream of waste that threatens city-dwellers with plague and pestilence unless it is safely carried away. Sewers convert that figurative stream into a literal one that flows below ground away from public view (and hopefully public smell). Your friendly neighborhood sewage collection system is not a magical place where gross stuff goes to disappear. It is a carefully planned, thoroughly tested system designed to keep the stuff we don’t want to see - unseen.