Speedwell Cavern entrance
It consists of a horizontal lead miners' adit (a level passageway driven horizontally into the hillside) leading to the cavern itself, a limestone cave. The adit is permanently flooded, resulting in Speedwell Cavern's (locally unique) feature: after descending a long staircase, the visitor makes the journey into the cave by boat (originally legged through by the guide, but now powered by an electric motor).
At the end of the adit, the visitor alights from the boat and walks into the cave to see the fluorspar veins, the stalactites and stalagmites, and the so-called "Bottomless Pit". This pit is 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 placed in the shaft over the years, at around 150 metres (490 ft).
A connection was discovered in 1999 between the Speedwell Cavern system and Titan, the largest natural shaft in the UK, which is 141.5 metres (464 ft) high.