Tourists who make the arduous trek to Machu Picchu, the ancient Inca citadel built high in the Andes of Peru, might have two big questions: Why build here, and how? A new study appears to have found the answers: Machu Picchu is built atop intersecting fault lines that form something like an X, reports Newsweek. The first obvious advantage to building in such a locale is that it already has an abundance of broken rocks in all kinds of shapes. This allowed the Incas to fit them together precisely without the need for mortar, notes Smithsonian. The fault lines also provided water by channeling rain and melting snow to the site, according to research presented at the Geological Society of America by Rualdo Menegat of Brazil Federal University. Similarly, the maze of underground fissures provided necessary drainage.
"Machu Picchu’s location is not a coincidence,” says Menegat in a news release. His team found that the Incas built other high, rocky settlements, including Ollantaytambo, Pisac, and Cusco, on top of fault lines, too. They may not have had today's understanding of what the faults represented, but Menegat says the Incas clearly had some inkling of their importance because they had a word for the fractures—quijlo. By building near them, the Incas could avoid lower settlements and their associated risks of avalanches and flooding. "No civilization could be established in the Andes without knowing the rocks and mountains of the region," Menegat tells Newsweek. Machu Picchu, believed to have been built around 1450, "is not an isolated case of Inca survival strategy in the Andes." (The biggest threat to the site today is tourism.)