9 'Unsolved' Mysteries That Have Been Solved

Why Did the Mayan Civilization Collapse?
We now know the Mayan civilazation collapsed from environmental factors. WIN-Initiative/Getty Images

For a long time, people have puzzled about one of the freakiest societal collapses in human history. Why did the Maya people abandon dozens of cities they'd built in the Yucatan peninsula in the 700s or 800s C.E., and allow what had been a highly developed civilization to turn into ruins?

Some have theorized that the Maya were probably defeated in battle by rival peoples or that the ruling class was overthrown in a peasant revolt. Others have advanced more outlandish explanations, such as an invasion by UFOs [source: Stromberg].

But in a study published in 2012, Arizona State University researchers, who analyzed archaeological data with an eye to figuring out environmental conditions in the Mayan heyday, found evidence to substantiate a theory first advocated by historian Jared Diamond in his 2005 book "Collapse." The Maya, the researchers discovered, had burned and chopped down so much of the forests that they had altered the land's ability to absorb solar radiation, which in turn made clouds and rainfall scarce. That exacerbated a naturally occurring drought, and caused erosion and soil depletion, which caused agriculture to fail. With less food available, workers were forced to leave the lowland cities to avoid starvation, and everything collapsed as a result [source: Stromberg].