Opus caementicium is a building material obtained by mixing quicklime, volcanic ash from Pozzuoli (pozzolana), water, and stone aggregates. Unlike modern Portland concrete, which weakens over time, Roman concrete in contact with seawater develops tobermorite and phillipsite crystals in its pores, which increase its strength over the centuries. Analysis of samples from Roman harbor piers (Jackson et al., American Mineralogist, 2017) revealed this self-repairing mechanism, unknown to engineering until the 21st century. The Pantheon in Rome (125 AD), with its 43.3-meter-diameter dome built of pozzolana concrete with no metal reinforcement, was the largest dome in the world for 1,300 years and still stands with no structural intervention. The formula was forgotten after the fall of the Roman Empire and was not reproduced until the 19th century. Modern Portland concrete, developed in 1824, is structurally inferior to Roman concrete in long-term durability in marine environments.