Centuries ago, a group of Aztecs setted in a valley in central Mexico. They built one of their cities on an island. The swampy land was graduually dried to make more room. Land was also needed to produce food. Garden islands were created. Farmers fenced in part of the shallow lake bed and filled the area with soil. Willow trees were planted at the corners to help hold the soil in place. Many crops grew well on the rich soil of the new islands.