The cloaca maxima started as streams carrying water from the Esquiline, Viminal and Quirinal hills before Tarquin hedged them in with walls. The channels, through which the muck flows, are about 3m square and were probably uncovered in the time of Plautus, who refers to them as canals. 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.
...Still the people murmured less at building the temples of the gods with their own hands, than at being transferred, as they afterward were, to other works, which, while less dignified, required considerably greater toil; such were the erection of benches in the circus, and conducting underground the principal sewer, the receptacle of all the filth of the city; two works the like of which even modern splendour has scarcely been able to produce.- Livy Book I