ancient Roman sewer, one of the oldest monuments in the Roman Forum. Originally an open channel constructed in the 6th century BC by lining an existing stream bed with stone, it was enclosed, beginning in the 3rd century BC, with a stone barrel (semicircular) vault. Its primary function was to carry off storm water from the Forum district to the Tiber, but in Imperial times large public baths and latrines were connected to it. Much of its original masonry has been replaced by concrete.