The Engineering Secret Behind Machu Picchu’s Stonework

The ancient stones of the Inca citadel Machu Picchu, in the heart of the Peruvian Andes, are among the remaining wonders of a vanished world—and its colossal walls are held together not by mortar, but by gravity.

Machu Picchu's builders used a type of "dry stone" masonry to join the massive blocks of stone almost perfectly, beside and on top of each other. The walls consist of thousands of interlocking blocks with irregular shapes as a result, and it's suggested that their construction made them more resistant to earthquakes, because the stones can move freely in place. But it's not clear if this resilience was the intention of the builders, or perhaps a lucky accident.

"I think that their intention was simply to build something that is efficient and functional," says civil engineer Nicolas Estrada Mejia of Peru's Los Andes Peruvian University, who has studied the ancient walls. "The result is impressive."

Machu Picchu's walls have stood for almost 600 years, although restoration work for tourism was carried out in the first decades of the 20th century. Headlines from the time suggested that Machu Picchu was a "lost city" that had been found by the American explorer Hiram Bingham. But it was never truly lost—locals had always known it was there—only abandoned due to a combination of factors related to the decline of the Inca Empire.

The no-mortar technique used to build Machu Picchu is sometimes called "ashlar" masonry, but technically that only refers to the square or rectangular blocks of stone in the walls. The Inca builders also used large "polygonal" stone blocks, each shaped exactly for the place in a wall where it was laid (such walls are found at other Inca sites, and one famous polygonal stone at the royal Inca capital of Cusco boasts 12 sides).

Inca buildings were designed to complement the landscape, says architectural historian Stella Nair at the University of California, Los Angeles. The Spanish colonists who came later built massive cathedrals and monumental palaces at places like Cusco, but they paled next to the grandeur of the Andes. "What the Incas did that was really smart is, rather than trying to make an architecture that alone is going to be impressive,” Nair says. “They instead made an architecture that was in dialogue with the vast impressive landscape."

Peru lies on the Pacific Ring of Fire, which makes the entire country prone to earthquakes. But while many Spanish colonial buildings in Cusco collapsed during a massive earthquake in 1650, the Inca walls there—and the walls at Machu Picchu—were unharmed by the tremors.

"We've got all this anecdotal evidence that Inca architecture endures earthquakes, but there's not been actual seismic research," she says. Civilizations in the Andes that predated the Incas had also used the same sort of masonry, and so it was likely that the Incas had adopted it from them, Nair says.

Research indicates that Machu Picchu was not, in fact, a "lost city," as Bingham put it. Instead, it seems to have been an estate for the Inca emperor and his courtly retinue, who may have spent part of the year there, explains Yale University archaeologist Richard Burger, an expert on the site and one of the editors of Machu Picchu: Unveiling the Mystery of the Inca. His research into the dates of burials at Machu Picchu suggests the structures were built in the middle of the 15th century, and probably for the powerful Inca emperor Pachacuti, who ruled from about 1438 until 1471. Pachacuti's reign marked a rapid expansion of the Inca Empire throughout the Andes, and the construction of Machu Picchu seems to have been part of that.

The Machu Picchu site is nestled on a saddle-like mountain plateau between two dramatic peaks: the "old peak" of Machu Picchu itself, which gives the citadel its modern name, and the "young peak" named Huayna Picchu, which rises to the north and features in most photographs.

The landscape is also unusually tropical and verdant for Peru. Cusco, for example, is located on a high and cold plain, which probably added to its appeal, Burger says. In addition, a spring fed by geological fissures that emptied onto the mountainous plateau was used in many of the area's sacred fountains.

Palace buildings, temples and the Intihuatana stone—aligned with the winter and summer solstices—show the Inca emperor was present at least some of the time, when he could take part in important religious ceremonies.

And while some people must have lived there throughout the year, their numbers swelled to up to 1,000 people when the emperor and his retinue came to stay. Some food, mostly maize, was grown on mountain terraces nearby, but most provisions would have been brought in, Burger says.

While the main walls at Machu Picchu are made without mortar, it was not because mortar was unknown to the Incas. A few small structures at the site use stones with mortar between them, or are covered with mortar in the style of stucco, Burger says. But the largest walls, made completely without mortar, are its most impressive feature.

"The Incas were certainly aware of earthquakes, and their buildings withstand earthquakes very well," he says. "In modern times, Machu Picchu has been heavily restored—but when there's an earthquake, only the restorations fall."