The majestic buildings of Ancient Rome have survived for millennia. But how did their building materials help keep colossal structures like the Pantheon (which has the world’s largest unreinforced dome) and the Colosseum standing for over 2,000 years?

Roman concrete in many cases proved to be more durable than its modern counterpart, which can deteriorate over decades. Now, scientists behind a new study say they have uncovered a mysterious ingredient that allowed the Romans to make their building material so durable and to build complex structures in difficult places such as docks, sewers, and earthquake zones.

A research team that included scientists from the US, Italy, and Switzerland analyzed 2,000-year-old concrete samples taken from a city wall at the archaeological site of Privernum in central Italy, which are similar in composition to other concrete samples found throughout the Roman Empire, reports CNN.

They found that the white pieces in the concrete, called lime clasts, gave the concrete the ability to heal cracks that formed over time. Previously, white pieces were not paid attention to, considering them to be evidence of careless mixing or low-quality raw materials.

“For me, it was really difficult to believe that ancient Roman (engineers) would not do a good job because they really made careful effort when choosing and processing materials,” said study author Admir Masic, an associate professor of civil and environmental engineering at the Massachusetts Institute of Technology. “Scholars wrote down precise recipes and imposed them on construction sites (across the Roman Empire).”

The new discovery could help make modern concrete production more sustainable, potentially shaking up society like the Romans once did.

“Concrete allowed the Romans to have an architectural revolution,” Masic said. “Romans were able to create and turn the cities into something that is extraordinary and beautiful to live in. And that revolution basically changed completely the way humans live.”