Concrete

Ancient Romans used a mixture of limestone, water, and volcanic ash to make concrete. This mixture and its curing technique produces a stronger and more environmentally friendly concrete than modern Portland cement. By 25 B.C., Romans had even developed a recipe for concrete to be specifically used for underwater work, which is essentially the same formula we use today. After the fall of the Roman Empire, the technology to make concrete was lost for nearly 1000 years before it was found again in the 14th century.