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Question: What secret Roman construction technique allowed the Colosseum and Pantheon—despite earthquakes, floods, and the ravages of time—to last more than 2,000 years?

Answer: Concrete

The ancient Romans were masters of engineering, constructing vast networks of roads, aqueducts, and monumental structures that were unrivalled in their time—and their remains continue to fill us with a sense of awe and wonder. Scholars have long puzzled how engineering marvels like the Colosseum and Pantheon could still be standing after two millennia. Now we know that the longevity of these structures can be attributed to one thing more than anything else—Roman concrete.

On the surface that makes no sense. Modern-day concrete structures are known to crumble after a few decades. Yet the Colosseum looks like it could still hold chariot races, several ancient aqueducts still deliver water to Rome today, and the Pantheon and its dome—the world’s largest unreinforced concrete dome—is in such pristine shape, you would swear it still has that "new temple smell."

Obviously, this was no ordinary concrete. And just as obviously, these were no ordinary buildings.

The Colosseum (also known as the Flavian Amphitheatre) was completed in 80 AD. It was four stories tall, close to 620 feet across at its widest point, and could hold between 50,000 and 80,000 spectators—and to protect all those Roman spectators from the sun, it had a retractable roof-like structure, known as the velarium. For its grand opening, the venue hosted more than 100 straight days of bloody gladiatorial contests, chariot races, recreations of ancient battles, and the slaughtering of some 9,000 wild animals. An iconic symbol of Imperial Rome, the Colosseum is the largest amphitheater ever constructed.

But in most aspects, the Pantheon—built about 40 years after the Colosseum—is even more impressive. The name itself—Pantheon—combining the Greek words for "all" and "gods," would indicate a religious function, but the actual function of this remarkable building remains shrouded in mystery.

Often imitated, never duplicated

Think about it—in the nearly 2,000 years since the Pantheon was built, no unreinforced concrete dome has ever been constructed on such a scale. How did ancient Roman engineers manage such a feat? For centuries, architects and engineers were baffled by the dome’s ability to sustain immense weight without visible pillars. Some half-jokingly speculated that there are two possible explanations: either gravity worked differently in Roman times, or they knew something that we still don’t. Seeing the Pantheon for the first time, Michelangelo said that it looked more like the work of angels, not humans.

As if it wasn’t enough for Roman engineers to build a dome so groundbreaking that it still can’t be duplicated, they added a circular window stretching 30 feet across at the dome’s apex called the oculus (or "eye"). Open to the heavens, the oculus floods the interior with natural light and acts like a sundial during the day. Now that just seems like showing off, doesn’t it?

The Pantheon is also one of the most imitated buildings in history (think the U.S. Capitol dome, Somerset House in London, or the library Thomas Jefferson designed for the University of Virginia). And after Michelangelo got over his shock, he drew directly from the Pantheon’s iconic form to design the dome of St. Peter’s Basilica in the Vatican. A Roman Catholic church since the 7th century, it’s the oldest building in the world that’s still in use today.

But we now know what it was that allowed architects to do away with load-bearing columns and introduce spacious domes in buildings like the Pantheon or why the Colosseum wasn’t reduced to rubble centuries ago during the region’s many earthquakes—concrete.

Scientists are chipping away at the secret to Roman concrete

The Romans didn’t invent concrete. But they came up with a unique "recipe" for concrete, one that is nothing like the concrete still in common use the world over. Researchers still studying Roman concrete have discovered that the ingredients appear to endow it with phenomenal resistance to degradation.

Most concrete in common use today is made up of portland cement—a combination of silica sand, limestone, clay, and chalk baked at a high heat and crushed into fine powder—along with pieces of rock or sand called aggregate. Mixing the rocky aggregate into the cement makes it stronger—and saves cement. Adding water then sets off a chemical reaction that binds these elements together. The aggregate in modern concrete is also carefully chosen so that no reactions take place after that first one, since future reactions could lead to cracks and weaken the concrete.

Roman concrete, on the other hand, took a completely different approach. It began with a simpler mix of quicklime made from baking and crushing limestone rocks. And for the aggregate, they used volcanic rocks of various types, which were abundant around Rome at the time. Instead of the inert rocky aggregates in modern concrete, the volcanic aggregates used by the Romans were highly reactive, so their concrete remained chemically active even centuries after the concrete hardened.

Normally, this would be a bad thing. But the Romans chose this aggregate on purpose, because the reactive volcanic minerals were making the Roman concrete stronger—they were reinforcing any small cracks that often form in cement and only get worse over time. Roman concrete, on the other hand, got stronger over time. The ongoing reactions gave it "self-healing" properties and made it much more resistant to cracking. And this wasn’t just a fortunate coincidence, the Romans knew this.

Thank the concrete for the staying power of the Colosseum and the Pantheon

You probably don’t even notice the concrete when you visit the Colosseum today. That’s because the most prominent feature is all that travertine limestone, but it’s there. What isn’t so evident is that concrete is what’s holding aloft all the amphitheater’s iconic archways. But the number one reason the Colosseum is still standing at all is because of concrete. It has a concrete foundation that is filled with dense lava rock and is some 40 feet thick. Without that robust concrete foundation, earthquakes would have reduced the Colosseum to rubble centuries ago.

But the most innovative use of concrete by the Romans was the dome of the Pantheon. It is highly doubtful that modern architects could come up with plans to build a dome of that size without reinforcements (like steel bars commonly used in modern concrete structures)—and even if they thought they could, the plans would be denied for violating so many civil engineering codes. But the Romans were such masters of engineering, what they did was gradually adjust the make-up of the aggregate in their concrete to make the structure of the dome lighter and lighter as it rose to the top.

The concrete was poured from the bottom up to the top into wooden frames that formed successive concentric circles. At the lowest and widest part of the dome, large blocks of heavy, dense basalt were mixed in the cement. And the final layer surrounding the oculus at the top contained airy pumice stone as an aggregate, which is so light it floats on water. The dome’s walls were also built much thicker at the bottom than at the top to more efficiently spread the massive load onto the sturdy base. Another trick those canny Romans came up with has to do with the curved interior of the ceiling—it’s covered in hollowed-out rectangles known as coffers that shaved 500,000 pounds off the weight of the dome, further lightening the massive load.

If Roman concrete is so good, why aren’t we using it?

Building infrastructure that will last 500 years or more instead of the current 100 years, along with Roman cement’s self-healing properties would seem to be a slam dunk. Not only that, but because Roman cement requires heating at far lower temperatures than Portland cement, it has the potential to offer massive reductions in the carbon footprint of cement production.

Apparently, the biggest obstacle to switching to a version of Roman cement is its lengthy curing time—up to six months to reach its full strength compared to the 28 days in the concrete we currently use, which is actually quite a bit stronger than Roman concrete. But there are ways to speed up the chemistry of Roman concrete. And it is feasible to incorporate other elements of the Roman recipe that would allow us to replace infrastructure far less often than we do now—and it is being looked at. The Romans clearly have a lot to teach us, but whether we listen is something else altogether. The sad reality is that the ancient world built structures because they wanted them to last forever, something totally at odds with construction goals in the modern world.

A few more ancient marvels that have modern engineers scratching their head:

• A lost city that was never really lost—Machu Picchu, often referred to as the "Lost City of the Incas," was never really lost, of course. The 15th-century citadel was just unknown to the outside world until its "rediscovery" by Hiram Bingham in 1911. Engineers still marvel at the Inca’s sophisticated dry-stone construction that fuses huge blocks without the use of mortar (too bad they weren’t in touch with the Romans!), along with terraced fields, and water distribution system. Whether is was a royal estate, astronomical observatory, or sacred religious site for Inca leaders, the exact purpose of Machu Picchu still isn’t known with certainty.


• Say "sexy woman"—Speaking of Incan marvels, Sacsayhuaman continues to intrigue scholars—especially how they managed to maneuver giant boulders of up to 120 tons each to fit together perfectly. For that matter, how did they get all those massive boulders to just outside Cusco from a quarry that was two miles away? Keep in mind that the Incas were unaware of the invention of the wheel.


• A great wall that really sticks—Begun as early as the 7th century BC and stretching for more than 13,000 miles, the Great Wall of China is one of the most impressive architectural feats in human history. Built to defend China’s northern borders against invasion, the wall wasn’t completely continuous, but rather a network of walls and fortifications along an east-to-west line constructed of stone, brick, tamped earth, wood, and more. Instead of Roman cement, China’s secret recipe for strengthening portions of the wall was … sticky rice! And no, the Great Wall of China cannot be seen from space with the naked eye.


• Where’s the garden, and don’t leave us hanging—Without definitive proof, it can’t be stated with certainty whether the Hanging Gardens of Babylon existed in the ancient world or not. But so many ancient texts attest to them, most historians and archaeologists think we’re just looking for them in the wrong place. Greek texts describe it as a series of terraces watered by an ingenious irrigation system. This advanced irrigation system—a chain pump—would have raised water from the Euphrates River to the gardens’ highest terraces, which would certainly have been a marvel of technology for the time.


• God bless you Nabateans, Mr. Rosewater—Petra, often called the "Rose Red City" for the color of its rock-cut architecture, may have been established as early as the 4th century BC as the capital city of the Nabateans. Abandoned the 7th century AD after several earthquakes destroyed much of the city’s water sources and buildings, it was another city that remained unknown to the Western World until it was "rediscovered" in 1812—this time by a Swiss explorer called Johann Ludwig Burckhardt. What most amazes contemporary scientists about Nabateans is how they were pioneers in hydraulic engineering, and were able to create complex water channels and dams to control the flash floods that regularly occurred in the region.


• Don’t go into the light!—An architectural and technological wonder of its time, the Lighthouse of Alexandria (also known as the Pharos of Alexandria) stood some 330 feet high on the island of Pharos to guide ships into the busy harbor of Alexandria, Egypt. The first known lighthouse in the world, it served as the model for lighthouses ever since. It is believed the lighthouse’s mirror could reflect light more than 35 miles out to sea. Some even claim it could burn enemy ships before they could reach the shore—well, let’s allow them that it might have temporarily blinded any sailors looking at it, but that’s it. Severely damaged by a series of earthquakes between 956 and 1323, the lighthouse eventually became an abandoned ruin—but its remains were used in the construction of the Citadel of Qaitbay in the 15th century on the same site. Ships may not be able to see it anymore, but you can still see portions of it in the structure of the citadel.

Visit the Colosseum during your free time to explore Rome on O.A.T.’s Italy’s Western Coast & Islands: A Voyage from Rome to Valletta adventure.