Long held notions that climate has been stable over the Yucatan peninsula and that today's climate is an accurate reflection of past climates here are being challenged today by a number of researchers. Both empirical and circumstantial evidence are offered for a prolonged and severe period of dessication in the Maya lowlands and its effects on soils, vegetation, lake levels and ancient Maya cultural processes, ca. 50 B.C. to 500 A.D. After centuries of steady and precocious growth and development, Late Preclassic Maya civilization in the drier northern two thirds of the peninsula abruptly collapsed, probably due to repeated crop failures and decreasing availabilities of potable water due to severe drought conditions. This is nowhere more apparent than at the predominantly Preclassic ruins of El Mirador - the largest known ancient Maya city - which was almost totally abandoned by A.D. 250. Thereafter, the development of classicism was confined to a small and better watered area in extreme northeastern Guatemala, northern Belize and southern Quintana Roo, which was defended from invaders from the north by a line of fortified sites.