In 1932, pilot George Palmer was flying from Las Vegas to Blythe, Calif., when he saw drawings sketched on the desert. Someone had scraped away the dark surface soil to draw three human figures, two four-legged animals, and a spiral.

Because they’re so large (the largest human figure is 171 feet long), they’d gone unnoticed until then. No local Native American group claims to have made them; radiocarbon dating places their creation between 900 BCE and 1200 CE.