A period of severe drought near the end of the 1st millennium C.E. likely sealed the fate of Lowland Classic Maya society, and a new study shows just how dry it was as the populations of the Maya Lowlands began to evaporate. The fall of the ancient Maya during the Terminal Classic Period (~800 - 1000 C.E.) is a commonly used example of how abrupt past changes in climate have contributed to the decline of ancient societies. While paleoclimate studies from the region indicate that the Classic Maya collapse occurred during an exceptionally dry period, it has not been clear just how dry that period actually was. Most climate data are limited to qualitative reconstructions - a period being simply wetter or drier than another, for example. In this study, Nicholas Evans and colleagues reconstructed the isotopic composition of water from Lake Chichancanab, Mexico, using sediment cores containing precipitated gypsum. They measured the triple oxygen and hydrogen isotope compositions of water molecules incorporated in the crystalline structure of gypsum deposited as layers on the lake bottom, as a proxy for past aridity. The authors found that during the Terminal Classic Period, annual rainfall in the Maya lowlands was reduced by nearly 50% on average, and up to 70% during peak drought conditions. Also, for the first time, the authors were able to measure a 3-8% decline in relative humidity compared to today. The study's results reveal the severity and duration of the drought experienced by Lowland Maya society and provide the quantitative data necessary to better understand its impact on Maya agricultural and sociopolitical systems.