The village of Fontaine-de-Vaucluse is built around the karst spring of the Sorgue, in a cul-de-sac valley at the foot of the Monts de Vaucluse. Made up of mostly permeable limestones, these mountains have favored the infiltration of water in depth and the appearance of underground rivers whose natural flow is blocked by deep harder and non-permeable rocks and which reappear as karst springs. Fontaine-de-Vaucluse is dominated by a 787-ft (240-meter) cliff where waters have dug a 1,010 ft (308 meters) deep siphon, including 731 ft (223 m) below sea level.