No one knows for sure how its creators achieved this level of accuracy—or why, given that ancient people weren’t lugging around stone slabs to navigate. We know Mesopotamians were savvy land surveyors, as other unearthed tablets include detailed renderings of farm fields and housing plots useful for things like tax assessment, says Augusta McMahon, University of Chicago professor of Mesopotamian archaeology and director of the school’s Nippur excavations. But the Nippur tablet covers a far larger area. Its creators likely used rudimentary surveying tools, like knotted ropes and rods, and maybe also a form of trigonometry for calculating angles. But making the map would still have required laborious, incremental measurements and patient tabulation.