A multi-year sequence of relatively mild droughts, apparently.
Relatively mild drought conditions may have been enough to cause the collapse of the Classic Maya civilisation, which flourished until about AD950 in what is now southern Mexico and Guatemala.
Scientists have long thought that severe drought caused its collapse.
But Mexican and British researchers now think that a sustained drop in rainfall of only 25-40% was enough to exhaust seasonal water supplies in the region.
"These reductions amount to only 25 to 40% in annual rainfall, but they were large enough for evaporation to become dominant over rainfall, and open water was rapidly reduced," Professor Eelco Rohling of Southampton University told the Press Association.
"Societal disruptions and abandonment of cities are likely consequences of critical water shortages, especially because there seems to have been a rapid repetition of multi-year droughts," he added.