The site is a cleft in the side of the Mells River that acts as a resurgence in winter and a swallet in summer. An excavation, 1974-78, began 21 m above and ended 2 m below river level. It showed that a high-level resurgence was active in the Middle Devensian and was buried under later deposits. Scattered finds of archaeological significance included Wooly Rhinoceros bones and a First Century bronze brooch. Water tracing experiments proved that the Sink is the upper end of an active underground oxbow to the Mells River, 2.5 km long. The cleft was developed along a neptunian dyke of Lower Lias age. Bubbling springs and ‘cave coral’ are briefly discussed.