Swallett Falls, lies just outside Foolow in the Peak District. Blink and you might miss it as you pass on the road to the plague village of Eyam. The falls drop about 50 feet in to what is a sink hole, the water comes in at the top and travels to the other end of the hole and drains away through the muddy floor. That said, when it rains heavily the water can back up and flood the entire floor of the sink hole to the depth of about 15 feet (at a guess).
You can see more from my Peak District photography here.