Speaking of rebar, did you know that the dome of the Pantheon in Rome is the largest structure ever built of unreinforced concrete? Did you also know that the fact that it is built of unreinforced concrete is the reason why it's still standing after nearly 2000 years? Rebar allows builders to build stronger structures using exponentially less concrete, but rebar also rusts inside the concrete matrix, producing cracks, fissures, and other nasty things that make buildings, bridges, and highway overpasses fall down. Everything built of reinforced concrete will eventually need to be razed or rebuilt, but the Pantheon will keep right on Pantheoning along.