http://en.wikipedia.org/wiki/Lamanai
Although most Mayan city-states had collapsed by or around the 10th or 11th centuries A.D., Lamanai, a medium-sized Mayan city that was founded around 1500 B.C., managed to survive another five centuries after the implosion of the Mayan civilization, finally reaching its own end at the beginning of the 17th century. The consensus among Meso-American archaeologists regarding Lamanai's unusual longevity points to the construction of large subterranean cisterns that were found throughout Lamanai. These cisterns were designed to capture and store large amounts of rain water, and were also used for food storage. It is widely believed that prolonged periods of drought led to famine and massive warfare among Mayan city-states. Lamanai's water storage capability likely allowed it to survive harsh drought seasons that devastated its neighbors.
