Rome’s main collector, the Cloaca Maxima, began as an open ditch in the late 7th–6th century BCE and was vaulted over time. By the mid-first century BCE—often placed around 60–50 BCE—Roman engineers reinforced crucial stretches with brick-faced opus caementicium, forming a stronger barrel vault that could resist collapses and move higher volumes from the Forum valley to the Tiber. Surviving segments and manholes still display those brick facings and concrete cores. The reinforcement mattered for