The barrows are primarily made of earth and silt, but also include granite stones, which likely formed burial chambers. Researchers took thousands of electrical resistivity readings by hand to map the site. The researchers used electrical resistivity mapping—think of a souped-up metal detector—to peek into the mounds by sending electrical currents into the ground to measure density. “The coexistence of standing stones embedded within the burial mounds and positioned atop them suggests scientifically interesting interactions.” “The presence of standing stones in these burial mounds remains a mystery to this day,” the authors wrote.