Museum displays of Roman building material tend to be badly labelled and quickly walked past. A broken lump of wall behind glass with a card reading “concrete, 2nd century AD” gives a visitor nothing. Which is a shame, because the cut face of that lump contains most of the answer to why Roman structures are still standing.

Here is what to look for when you find one.

The wall is a sandwich, and the pretty part is the formwork

The single most common misreading is treating the visible facing as the wall. It is not. Roman walls of this type consist of a thick core of mortar and rubble, faced on both sides with a thinner skin of stone or brick that served as permanent shuttering during construction and as a finished surface afterwards.

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.