Why Machu Picchu has not slid off its mountain after centuries of rain

Machu Picchu sits almost 8,000 feet up on a knife-edge ridge, with about 200 heavy stone buildings on top and over six feet of rain every year. Terraces stopped the slopes from sliding all at once, but water soaking in from underneath was the real threat. Thicker walls would only trap more of it. So the Incas gave the water somewhere to go. Under the terraces they built down first: rock, gravel, sand, then a thin layer of soil. Under the main plaza, about three feet of stone chips left over from carving the buildings. And 129 outlets carry the water out through the walls. Sources: • Wright (waterhistory.org) — https://waterhistory.org/histories/machupicchu/machupicchu.pdf • UNESCO WHC 274 — https://whc.unesco.org/en/list/274/ • Retaining wall — Wikipedia — https://en.wikipedia.org/wiki/Retaining_wall