Sewer

A sewer is a cavernous tunnel or a series of pipes through which effluent and storm water generally flow.

Sewers such as the London sewers have been used throughout the 18 and 20th centuries by an array of species for purposes other than those which sewers are usually used for.

Apocrypha
On Tersurus the Master journeyed through Terserus's sewers several times. (DW: The Curse of the Fatal Death)