One of many fascinating archeological spots left behind by the Incas is Moray, near Cuzco, Peru. There are four amphitheater like series of concentric circles terraced into the ground at an elevation of 11,500 feet. Though it is debated, a popular theory is that this was an agricultural testing station of micro-climates, since the temperatures varied by as much as 27-60 degrees Fahrenheit within the stations from top to bottom and they included a sophisticated irrigation system. It might be compared to a modern day greenhouse. Due to design or location, the structure never floods, even during the rainy season. The Incas studied and used hundreds of varieties of maize and thousands of potato varieties, leaving behind a priceless legacy for mankind, especially in our modern age of climate change.
Additional Information: here, here, and here. Additional photo here.
UPDATE 8/3/2011: A new book by a civil engineer from Boulder, Ken Wright, claims to have disproven the Moray site as having an agricultural purpose. He and his wife, Ruth, have researched the area and came to the conclusion that it was a religious site for water worship.