Sewer

A sewer is an artificial conduit or system of conduits used to carry and remove sewage and to provide drainage. Human sewers are usually pipelines that begin with connecting pipes from buildings to one or more levels of larger underground horizontal mains, which terminate at sewage treatment facilities. Vertical pipes, called manholes, connect the mains to the surface. Sewers are generally gravity powered, though pumps may be used if necessary. It should be noted that sewage pipelines are unconnected to other pipelines such as those for potable water.

Relevance
During their assault on Côte d'Azur, Spartan Blue Team used the sewer systems to navigate through the city undetected.

It is unknown what the Covenant analog to a sewer is, if they even have one.