The fact that the Roman Pantheon still stands is equal parts amazing and confusing. Built in Rome in the 2nd century AD, the Pantheon is a massive concrete building capped by an impressive 142-foot-high dome—the largest in the ancient world.

Made entirely out of concrete, without the reinforcing support of structural steel, no modern engineer would dare attempt such a feat, says David Moore, author of The Roman Pantheon: The Triumph of Concrete. “Modern codes of engineering practice would not permit such mischief.”

And yet for nearly 2,000 years the Pantheon has stood, weathering earthquakes, Barbarian invasions and the persistent onslaught of Mother Nature.

For years, researchers have figured there must be something special about the concrete used to build the Pantheon and other Roman monuments that lend them such longevity. Many scientists have pointed to the practice of including volcanic ash in the concrete mix, as Erin Wayman wrote for Smithsonian in 2011.

In a new study, researchers drilled down into the chemistry of Roman concrete to find out what makes it so resilient. As suspected, the key ingredient is the specific blend of limestone and volcanic ash used in the mortar, says Gail Silluvan for the Washington Post.

Mixing mortar according to the recipe of 1st century Roman architect Vitruvius, the scientists' analyses unveiled that the mortar included “dense clusters of a durable mineral called strätlingite.”

“The crystals formed because of a reaction that took place over time between the lime and volcanic matter in the mortar,” says Sullivan, and “helped prevent the spread of microscopic cracks by reinforcing interfacial zones, which researchers called 'the weakest link of modern cement-based concrete.'"

Sullivan says that the Roman technique actually has some benefits over modern mixes: