In a waist-high trench alongside Spain’s national Highway 1, a dozen volunteers wearing rubber gloves brush tan clay from crumbling human bones. Their knees rest on foam cushions, and a white tent shades them from the summer sun. It’s July 2011—a full 75 summers after Spain erupted in the Civil War that put the bones of 59 civilians in the ground here. A few steps away from the trench, volunteers hold microphones up to the murmuring mouths of elders from the town of Gumiel de Izán, in the north-central region of Castilla y León. The elders, who harbor memories of...