The endurance of Roman architecture is largely due to 'Opus caementicium,' or Roman concrete. Unlike modern concrete, the Roman version utilized volcanic ash (pozzolana) found near Mount Vesuvius. This ash created a chemical reaction that allowed the concrete to set underwater and actually become stronger over time through the growth of rare minerals. This innovation allowed the Romans to build massive domes, vaults, and harbors that stood for millennia. The most famous application is the dome of the Pantheon in Rome, built by Emperor Hadrian. It remains the largest unreinforced solid concrete dome in the world, nearly 1,900 years after its completion. The recipe for this durable material was lost after the fall of the Western Roman Empire and was not rediscovered until the late 18th century, illustrating the high level of technological sophistication that characterized the Roman peak.