Lake Nemi sits in the throat of an extinct volcano twenty-five kilometers southeast of Rome. The crater walls rise three hundred meters above water so still and so dark that the Romans called it speculum Dianae - the mirror of Diana. The grove of Diana Nemorensis ran along the northern shore. Pilgrims climbed up from the Appian Way and along the lake's edge to leave votive offerings - small bronze figures, terracotta lamps, anatomical models of body parts they hoped the goddess would heal. The town of Aricia, modern Ariccia, sat on the crater rim above. It was one of the oldest Latin settlements, allied with Rome before Rome quite knew what it was. The grove was the older institution. Rome was the upstart.

In the heart of the sacred grove stood a particular oak. No one was permitted to break a branch from it - except, by ancient rule, a runaway slave. If he managed to snap off a single golden bough, he earned the right to fight the grove's priest, the Rex Nemorensis, the King of the Wood. The fight was to the death. If the slave won, he became the new priest. Then he waited, day and night, for the next runaway to come for him. Strabo wrote that the priest paced the grove with his sword always drawn. Frazer used this brutal job description as the opening of The Golden Bough because it asked the question that obsessed him: why would any society organize itself around such a thing? The answer, he thought, ran through every myth he could find.

The grove's tutelary tree was the oak, and the oak in ancient Italy belonged to the lightning gods - Jupiter Latiaris on the mountain above, and through him to the older sky powers. To be struck by lightning was to be touched by the divine, which is why fulgurites and lightning-blasted trees were buried with ritual care. Diana of Nemi held the grove on behalf of those older forces. When the sacred oak was struck - and an oak that old, in a place that exposed, was struck regularly - the strike was read as a renewal of the goddess's authority, and a warning. Caligula visited in 40 AD with two enormous pleasure barges built specifically for the sacred lake. Hearing that the current King of the Wood had reigned an unusually long time, he sent his strongest slave into the grove to challenge him. The succession, the emperor felt, had been delayed.

Ariccia kept its small reputation through the centuries that followed. The Via Appia ran through the town, and travelers leaving Rome stopped here for the night before the long descent toward Naples. Pope Alexander VII of the Chigi family loved the place and brought Gian Lorenzo Bernini up the hill with him in the 1660s. Bernini rebuilt Palazzo Chigi, the round-domed church of Santa Maria Assunta, and the piazza that ties them together. Walk that piazza today and you can feel his fingerprints in the symmetry, the way the eye is pulled from the palace door to the church facade and back. In 1854 the Ponte di Ariccia opened - a three-tier stone viaduct three hundred meters long and sixty-nine meters high, looking exactly like a Roman aqueduct because that was the point. It still carries the road across the volcanic ravine.

The grove is gone. Centuries of woodcutting, then the Roman drainage of Lake Nemi's sister lake for fish, then in 1944 the deliberate destruction by retreating German forces, who blew up Mussolini's drained-out hangar and burned the salvaged remains of Caligula's barges. Foundations of Diana's sanctuary are still traceable along the northern shore. In Ariccia itself you can sit in the piazza, eat porchetta - the town's other claim to fame, herb-roasted pork carved off the bone with a long blade - and feel the strangeness of the place. The grove that Frazer made famous is one ridge over. The volcano underneath is sleeping but not dead. Lightning still finds the oaks on the crater rim.