Royston Cave

Royston Cave is a small artificial cave located in Katherine's Yard, Melbourn Street, Royston, England. It is located beneath the crossroads formed by Ermine Street and the Icknield Way. It is protected as both a scheduled ancient monument and Grade I listed building. It has been speculated that it was used by the Knights Templar, who founded nearby Baldock, but this is unlikely, despite its enormous popular appeal. There are numerous theories about the Cave covering Freemasons and Templars as well as possibilities that the Cave was a prison or an anchorite cell. However, none of these theories have enough hard evidence to warrant their being adopted by the Cave Trust. It is open to the public in the summer months on Saturday, Sunday and Bank Holiday afternoons between Easter and October.

Royston Cave is a circular, bell-shaped chamber cut into the chalk bedrock. It is 8 metres (26 feet) high and 5 metres (17 feet) in diameter with a circumferential octagonal podium. The origin of this chamber is unknown. This cave is unique in Britain - if not the world - for its numerous medieval carvings on the walls; comparable examples exist only in the former Czechoslovakia and the former Palestine. Some of the figures are thought to be those of St. Catherine of Alexandria, St. Lawrence and St. Christopher.

Royston Cave has been the source of much speculation, although it is hard to determine much about its origin and function.

Although the origin of the cave is unknown, the story of the rediscovery is very well known. In August 1742 a workman dug a hole in the Butter Market to build footings for a new bench for the patrons and traders. He discovered a buried millstone and dug around it to remove it. He found a shaft leading down into the chalk.

...
Wikipedia