<human>:  How did the Great American Dust Bowl start?
<bot>: The Dust Bowl was caused by a combination of a drought and overgrazing. The drought was caused by the collapse of the Ogallala Aquifer, which is a huge underground layer of water that lies under the Great Plains. Overgrazing was caused by the over-extraction of the grasslands for agriculture. The Dust Bowl was a major disaster for the United States, and it was a major reason for the Great Depression.