How Was Machu Picchu Built?

They built a city on a razor-sharp ridge 2,400 meters above sea level, where it rains nearly 2 meters a year. Instead of blocking the water, the Incas first laid out paths for it to drain underground. First, stone debris at the base of the retaining walls, then gravel, and then sand-mixed soil. It's like they took the same principle of putting gravel at the bottom of a flower pot and applied it to an entire mountain. What's even more surprising is that the debris was actually waste from cutting stone. There are calculations suggesting that six-tenths of the construction materials went underground this way. This is why the retaining walls remained almost intact even after being buried in the jungle for 400 years after abandonment.