The actual word cement can be traced back to Roman masonry known as opus caementicium. This material was comprised of burnt lime, volcanic ash, and crushed rock. The Roman Architects found that by adding volcanic ash, cement hardened underwater and was incredibly resistant to water erosion. The Roman aqueducts, harbors, and Pantheon were constructed from this early cement. After the Roman Empire fell, architectural skill did as well. Instead of using volcanic ash, they used carbonation of lime which was a much slower process. It was used to build great medieval cathedrals Durham, Rochester, and many others. However, it was not until the Industrial Revolution that modern concrete was rediscovered through the construction of lighthouses.