Below-ground piped municipal sewer systems act as collectors of urban-associated microbial communities. For instance, these systems receive a constant flow of microbial communities from the human body, household food waste, and industrial waste streams. Despite sewers being a relatively new habitat and one with a constant flux of immigrants, a common microbial community structure has emerged. The origins of this new community and why the community structure is so predictable remain a mystery. Here I will discuss what is known about the sewer microbial community and how community coalescence theory could shed light on this hidden world beneath our feet.