Sources

1) http://www.history.com/topics/inca

2) http://www.history.com/topics/aztecs

3) http://www.history.com/topics/maya

4)http://www.aztec-history.com/olmec-civilization.html

5)http://www.britannica.com/topic/Olmec

6)http://www.allabouthistory.org/aztec-civilization.htm

7)http://www.livescience.com/41781-the-maya.html

8)http://www.pbs.org/wgbh/nova/ancient/lost-inca-empire.html

9)http://www.newworldencyclopedia.org/entry/Aztec_Civilization

10)http://www.aztec-history.com/mayan-civilization.html