A cave situated on the edge of a housing estate and golf course in Bristol. The cave consists of some short climbs, and a few chambers, the last of which is a very tall rift chamber with a lake that changes height by as much as 20 metres. Side passages contain impressive crystal formations. The cave was formed by rising geothermal water (the only known example of a hydrothermal cave in the UK or Ireland), making it far older than other caves in the area, at around 190 million years old.
It also has the honour of having the first known use of ropes to descend a cave in 1669 by Captain Samuel Sturmy, the oldest published survey in the UK or Ireland, dating to 1683, as well as the earliest recorded caving fatality in UK or Ireland, of Thomas Newman on 17 March 1775, who fell down the main pitch while attempting to plumb its depth. Though there were earlier recorded deaths in caves, these were not due to caving, such as the Viking massacre at Dunmore Cave in County Kilkenny, Ireland, in 928 AD. Doubtless there were other fatalities during the times that the Romans mined minerals from various caves, and also fatalities of earlier cave dwellers. (Long Hole in Cheddar Gorge had an elevation survey done before 1680, but it was not published until later.)
There is a description (PDF) and partial survey online. There is also a dedicated website, which includes the original survey and current survey, photographs, and further details. Access is tightly controlled by the UBSS, BEC and WCC on behalf of the local council.