The area around Cuzco has a very short growing season because it is located at 11,000 foot elevation. As the Ancient Incan population increased, so did the need for more food. Incan ruler Sinchi Roca (1228-1258) decided on two ideas, both which were extremely labor intensive.
The first was to build agricultural terraces. Millions of hours of labor were required for llamas and humans to carry the rocks and soil up the mountains. Once in place, water had to be carried up the mountain, and the terraces needed to be weeded, raked, and seeded.
The second idea was draining the marshland. This required steep channels to be cut into the earth with rudimentary tools. Draining the marshland actually required more labor than building the massive terraces.