Speedwell Cavern is one of the four show caves in Castleton, Derbyshire, England. The cave system consists of a horizontal lead miners' adit (a level passageway driven horizontally into the hillside) 200 metres below ground leading to the cavern itself, a limestone cave. The narrow adit is permanently flooded, so after descending a long staircase, access to the cave is made by boat. At the end of the adit, the cavern opens up with fluorspar veins, stalactites and stalagmites, and the so-called "Bottomless Pit". This chamber has an underground lake with a 20 metres (66 ft) high waterfall and an extremely deep vertical shaft, now choked to within 20 metres (66 ft) of the surface by rock spoil dumped by miners. The original depth of the shaft has been estimated, from the amount of spoil place