Fosse Dionne Yonne

Some 200L of water per second gushes from Fosse Dionne, a natural spring near the centre of Tonnerre that was sacred to the Celts and whose vivid blue-green tint hints at its great depth. Legend has it that a serpent lurks at the bottom. Forming a picturesque backdrop behind the great circular pool are a mid-18th-century washing house, a semicircle of ancient homes and a steep forested hillside.