Roman concrete has been underwater for two thousand years in some locations and is stronger now than when it was poured. This is not a figure of speech. The concrete used in Roman harbor structures — the piers, breakwaters, and seawalls built along the Mediterranean coast during the Republic and Empire — has been studied by geologists and materials scientists who have found that it has been gaining strength over time rather than degrading, a property that modern Portland cement concrete does not share. Understanding why this happens has become one of the more productive intersections of archaeology, geology, and materials science in recent decades, and the answer reveals something important about Roman empirical knowledge and its limits.