The cloaca maxima
was the sewer system built in the sixth or seventh century B.C., by one of the kings of Rome to drain the marshes in the valleys between the hills into the Tiber River. The cloaca maxima began under the Argiletum, the street of booksellers, then it went under the forum and then it joined up with streams carrying water from the Esquiline, Viminal and Quirinal hills to finally discharge into the Tiber. The cloaca maxima itself was the large central canal. It is because of this drainage (and probably land-fill), that the area among the hills became habitable and provided the space for the forum Romanum.
See: "The Anatomy of Rome from Capitol to Cloaca," by Emily Gowers The Journal of Roman Studies Vol. 85, (1995), pp. 23-32.