Paul Crabtree

Modern concrete starts falling apart in a few decades. Roman concrete has been sitting in seawater for 2,000 years and getting stronger. The Romans figured out that volcanic ash changed everything. They dug it from around the Bay of Naples, near the town of Pozzuoli, mixed it with lime and chunks of rock, and got a material that could set even underwater. The architect Vitruvius wrote about it in the first century BC. Pliny the Elder claimed that once it hit the sea, it became a single mass of stone the waves couldn't break. He wasn't exaggerating. Roman harbor walls and piers built in the sea are still standing today, while modern concrete in the same conditions cracks and crumbles. For a long time nobody knew why. In 2017, a geologist named Marie Jackson studied samples from ancient Roman piers and found the answer. Seawater soaking into the concrete was reacting with the volcanic ash and growing rare mineral crystals inside it. The ocean that destroys our concrete was actually reinforcing theirs. Then in 2023, MIT researchers found something else. Roman concrete is full of little white chunks of lime that scholars had long assumed were just sloppy mixing. They weren't. The Romans were likely mixing with hot quicklime, and those chunks acted as a built-in repair kit. When a crack formed and water seeped in, the lime dissolved and filled the crack back in. Self-healing concrete, two thousand years before anyone used that phrase. The Romans built on a scale that shows they trusted it completely. Herod the Great had volcanic ash shipped all the way from Italy to build an artificial harbor at Caesarea in Judea. Hadrian's Pantheon in Rome, finished around 125 AD, still has the largest unreinforced concrete dome on earth, about 142 feet across. No steel. Nothing holding it up but the concrete itself. After the Western Empire collapsed, the know-how faded. Medieval Europe mostly built in stone and timber. Modern Portland cement wasn't patented until 1824, and it still can't match what the Romans were pouring into the sea before the birth of Christ.

Andenes are the stepped agricultural terraces the Incas and earlier Andean cultures carved into steep mountain slopes, and they are one of the defining features of the Sacred Valley. The word comes from the Spanish "andén," meaning platform or step, and it is often cited as one possible root of the name "Andes." Each terrace is a flat platform held up by a retaining wall of fitted stone, stacked in tiers up a hillside like a giant staircase. They were carefully engineered rather than just piled earth. A well built andén had large stones at the base for drainage, then layers of gravel and sand, and finally a deep layer of fertile topsoil often carried up from the valley floor. This structure let water filter through slowly instead of washing the soil away. The Incas built them for several reasons. They turned steep, useless slopes into level farmland in a region with almost no flat ground. The stone walls held soil in place against heavy mountain rains and stopped erosion. The drainage layers managed water and prevented landslides and waterlogging. And the walls created microclimates, absorbing heat by day and releasing it at night to protect crops from frost, while different elevations allowed a wide range of crops such as maize, potatoes, and quinoa. The best examples in the Sacred Valley are at Pisac and Ollantaytambo, where terraces sweep up the mountainsides in long curved and straight tiers. Moray is a special case, a set of deep concentric circular terraces that many researchers believe served as an agricultural testing ground, since each ring sits at a slightly different temperature. Many of these terraces are still farmed by local communities today, more than five centuries after they were built, which shows how well the drainage and soil engineering was designed.