The Disappearance of the Maya
The most likely explanation is that the Mayan system of agriculture, which relied upon a system of clearing rain forest and burning the vegetation to enrich the soil, broke down under the weight of their population. As soon as they surpassed a critical mass, the fields couldn't be left fallow long enough for them to return to fertility. This led to a downward spiral; fields were overworked and the carrying capacity of the Mayan system of growing corn simply collapsed.
The lesson of the classic-era Maya's demise could be interpreted as a very stark reminder that civilizations that use vital resources at an unsustainable rate face the same fundamental issues. However great the architecture, learning, or history of a people, if there is no food, everything comes to a grinding halt and the balance of nature returns.