ROME, ITALY—A small hole in the ground hidden by bushes has led scholars to the biggest tunnel yet found beneath Hadrian’s second-century A.D. villa in Tivoli. The intact tunnel, dubbed the “Great Underground Road,” was large enough to allow carts and wagons to pass, so that servants could carry food and firewood from one part of the emperor’s vast palace to another. “The underground network helps us to understand the structures that are above ground,” said archaeologist Vittoria Fresi.