It's one of the most prominent societal collapses in human history. The Mayans seemingly abandoned their complex civilization and disappeared into the Central American jungle. For centuries, people puzzled over the disappearance, theorizing everything from an internal peasant revolt, to conquest by an outside and unknown people, to a UFO holocaust.

It wasn't until 2005 that a legitimate theory was put forward to explain what happened, a theory confirmed in 2012. The Mayan civilization collapsed due to a self-created environmental disaster. The Mayans chopped down too many trees, which reduced the land's ability to absorb solar radiation. This made rainfall more scarce, which caused a crippling drought. The Mayans abandoned their land not due to aliens or revolt, but to find food.