The Classic Mayan civilization, located in the lowlands of present-day southern Mexico and Central America, began a precipitous decline around AD. 900. The majority of the people abandoned their great urban centers, buildings were no longer kept up, and carved monuments-a hallmark of the civilization-were no longer created. Various theories about this collapse have been explored, such as invasion from foreigners, natural disaster disease, failure of agricultural techniques, and internal revolt, although it is a combination of factors that probably constitutes the ultimate explanation.

When one looks at the remains of a city, it is difficult to determine why a building collapsed. Whether caused by natural disaster, natural decay. or structural weakness, the rubble and remains can look very similar. There is no historical record of earthquakes in the central Mayan lowland area at the end of the Classic period, although some evidence suggests that Mayan centers in the southern lowlands experienced earthquakes. Hurricanes can destroy significant quantities of crops and are therefore another possibility; however, the effects of both hurricanes and earthquakes are generally local and not so widespread as to cause the abandonment of the entire lowlands. An ill-timed natural disaster certainly could have compounded other problems brewing in Mayan civilization.

The possibility of drought throughout the area has also been considered and to date stands as a likely contributor to the collapse Lake Chichancanab, located in the central Yucatán Peninsula, is the largest closed-basin lake in the Yucatán. Studies of lake-bottom sediment revealed that between AD. 800 and AD. 1000. the lake experienced its driest period in the past 8,000 years, with aridity peaking in AD. 922. These are useful findings indeed; however Lake Chichancanab represents only one area in the Mayan region and evidence for drought in other regions needs to be examined.

Agricultural collapse is most likely to have been another important factor. Research done in 1985 shows an accumulation of silt in the lakes of the Petén region at the end of the tenth century An increase in sediment signifies an erosion of soil brought on by deforestation. Slash-and-burn agriculture-a style of farming that requires clearing new plots of forest land every two years-was probably the Maya's primary means of subsistence. It is a system of agriculture requiring large amounts of land and resulting in deforestation. It is possible that the Maya simply tapped their natural resources until none that were accessible were left

As populations in the lowlands grew, additional methods of farming were developed, but the number of people may have outweighed the capacity of the land, resulting in a food shortage. Population studies of the Mayan area continue, but current general consensus puts the numbers well into the millions. Some scientists say there was a conversion from diverse agricultural management to the exclusive cultivation of maize, which concluded with the end of the Classic period. The shift reflects the change in Mayan culture and in how the people were managing their land, although it does not explain why the change came about. Once the culture's resources were stressed, people were more vulnerable to the hardships of natural disaster, poor health, and social chaos.

In addition, pottery, architecture, and sculpture in some Mayan centers changed significantly at the end of the Classic period suggesting a takeover by an outside or surrounding group. Clear evidence of this kind of invasion has been found at Altar de Sacrificios and Seibal, although few other sites show such blatant signs. It is not understood exactly where the invaders came from, but most likely they were from the surrounding Mexican states. Also supporting this theory is the fact that the seafaring Putún Maya of Tabasco, Mexico, were in power in the Yucatán peninsula by the postclassic period. Lowland centers whose success depended on internal trade routes were no longer in the heart of the economic and political action; as the Putún gained power, the focus of trade seems to have moved toward the coasts. This could have pushed people to abandon their homeland, following the center of commerce.