In northwest Saudi Arabia, scattered across an area twice the size of Portugal, archeolog|sts and aerial surveyors have identified more than 1,000 roughly built, low, rectangular stone structures that date back 7,000 years to an era when today's deserts were savannas. These mustatils-"rectangle" in Arabic-have been long-known to regional tribes, and in 2018 archeologists began to investigate and excavate. Discoveries of animal bones and horns point toward ritual purposes. The great number of mustatils may be evidence of population and social organization. But why are there so many-and in so many different places? While no two are quite the same in length and width, all are close in height and shape. Amid more questions than clues, archeologists continue to dig.