Mur de la Peste
Attempts to stop the Great Plague included an Act of Parliament (of Aix) that levied the death penalty for any communication between Marseille and the rest of Provence, and a plague wall (Mur de la Peste) was erected across the countryside. The wall was built of dry stone, 2 m high and 70 cm thick, with guard posts set back from the wall.
Remains of the wall can still be seen in different parts of the Plateau de Vaucluse:
One section is 2 km east of Fontaine-de-Vaucluse. The GR97-6 leads in to the wall from Fontaine-de-Vaucluse and from Lagnes, about 5 km south of the wall. There's a nice hike (described on the Fontaine-de-Vaucluse page) that follows long sections of the wall, some of it in pretty good shape, but not as high as it once was.
Another section of the wall accessible by road or hiking runs west from the Col de la Ligne on the D15 road. The D15 runs between Murs (about 8 km south of the pass) to Monieux, at the top end of the Gorges de la Nesque (about 12 km north). You can also drive in on the D5 road from the village of Méthamis, which is about 6 km south of Villes-sur-Auzone. There's an information panel about the Mur at the Col de la Ligne.