The Segmental Arch
Like almost all of the engineering feats we've listed, the Romans didn't invent the arch -- but they sure did perfect it. Arches had been around for nearly two thousand years before the Romans got a hold of them. What Roman engineers realized (quite brilliantly, as it turned out) was that arches need not be continuous; that is, they don't have to span a gap in one go. Instead of trying to cross gaps in one great leap, they could be broken up into several, smaller sections. Turning an arch into a perfect semicircle wasn't necessary so long as each section had struts underneath. That's where the segmental arch came in.
This new form of arch-building had two distinct advantages. First, because the arches could be repeated rather than having a single stretch across a gap, the potential distance for a bridge span could be increased exponentially. Second, because less material was required, segmental arch bridges were more amenable to the flow of water underneath them. Instead of forcing water through a single small opening, water under segmented bridges could flow through freely, reducing both danger of flooding and the amount of wear on the supports.