The Shell Grotto consists of a winding subterranean passageway, about high and in length, terminating in a rectangular room, referred to as the Altar Chamber and measuring approximately. The excavations are entirely underground. Steps at the upper end lead into a passage about wide, roughly hewn out of the chalk, which winds down in serpentine fashion until it reaches an arch, the walls and roof of which here onward are covered in with shell mosaic. The arch leads to what is known as the Rotunda, a central circular column, meeting at the farther side at the Dome - a shaft rising to the surface, capped to allow some daylight into the structure. The plan of the sub-base of the Dome is triangular, equilateral, and with an arch in the centre of each side. The two arches in the sides are those leading from the Rotunda, whilst the arch in the base leads into the Serpentine Passage. This passage, with its curving walls and over-arching vaults, is rich in mosaics of varied design. At the end of the Serpentine Passage, a further arch leads into the Rectangular Chamber. Here the decoration takes on a more formal and geometric character, but still finely drawn and executed. The subjects are chiefly star and sun shapes. The focal point, the "altar", is the arcuate niche which faces the entrance arch. The purpose of the structure is unknown and various hypotheses date its construction to any time in the past 3,000 years. Hypotheses include: it was an 18th or 19th-century rich man’s folly; it was a prehistoric astronomical calendar; it is connected with the Knights Templar or Freemasonry. No scientific dating of the site has been carried out. The most frequently used shells throughout the mosaic – mussels, cockles, whelks, limpets, scallops, and oysters – are largely local. They could have been found in sufficient numbers from four possible bays: Walpole Bay in Cliftonville; Pegwell Bay especially at Shellness Point, Cliffsend, near Richborough; Sandwich Bay, Sandwich; and Shellness on the Isle of Sheppey. The majority of the mosaic is formed from the flat winkle, which is used to create the background infill between the designs. However, this shell is found only rarely locally, so could have been collected from shores west of Southampton, where it is abundant. Attached to the grotto is a modern museum, gift shop, and cafe.
History
There are conflicting accounts of the grotto’s discovery, although most agree on a date of 1835. The earliest reference to the discovery appears in an article in a predecessor of the Kentish Mercury of 9 May 1838: It has remained in private ownershipever since. In 1932, the then new owner took over the grotto, and soon afterwards substituted electric lighting for the gas lighting that, over the decades, had blackened the once-colourful shells. Cleaning trials show that in the majority of the grotto the shells have lost their colour under the dirt and are white. The structure has also suffered the effects of water penetration, though was removed from the Heritage at Risk Register in 2012 after a five-year conservation programme, carried out in partnership with English Heritage. A scheme to sponsor replacement mosaic panels – The Roundel Project – was established in 2012. The Friends of the Shell Grotto was formed in 2008 and is a not-for-profit trust established to promote, conserve, and preserve the grotto as a unique historical monument.
