The Rise and Fall of the Mayan Empire: "Sever and co-worker Dan Irwin have been looking at satellite photos and, in them, Sever spotted signs of ancient drainage and irrigation canals in swamp-like areas near the Mayan ruins.
Today's residents make little use of these low-lying swamps (which they call 'bajos,' the Spanish word for 'lowlands'), and archeologists had long assumed that the Maya hadn't used them either. During the rainy season from June to December, the bajos are too muddy, and in the dry season they're parched. Neither condition is good for farming.
Sever suspects that these ancient canals were part of a system devised by the Maya to manage water in the bajos so that they could farm this land. The bajos make up 40% of the landscape; tapping into this vast land area for agriculture would have given the Maya a much larger and more stable food supply. They could have farmed the highlands during the wet season and the low-lying bajos during the dry season. And they could have farmed the bajos year after year, instead of slashing and burning new sections of rain forest."