Subterranean passageway covered in seashells
The Shell Grotto is an ornate subterranean passageway shell grotto in Margate, Kent, England. Almost all the surface area of the walls and roof is covered in mosaics created entirely of seashells, totalling about of mosaic, or 4.6 million shells. It was claimed to have been "discovered in 1835", but its age and purpose remain unknown. The grotto is a Grade I-listed building and open to the public.
Overview.
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 grotto is 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 gothic-style 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 was a meeting place for sea witchcraft; and it is connected with the Knights Templar or Freemasonry. Since the 2007 discovery of a domed cave under the Palatine Hill in Rome with shells, mosaics and marble in similar patterns to those in the Rectangular Chamber in Margate, some credence has been given to the theory that the Shell Grotto could have been created by the Phoenicians in the second half of the first millennium BCE, when they were founding many colonies from their base in Carthage; however the gothic style of the arches would be a first for a pre 12th century arcade.
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 and gift shop.
Cultural depictions.
The grotto is featured in the book Elijah's Mermaid (2012) by Essie Fox.
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:
 ;templatestyles src="Template:Blockquote/styles.css" / ;Belle Vue cottage, a detached residence, has been lately been purchased by a gentleman, who, having occasion for some alterations, directed the workmen to excavate some few feet, during which operation the work was impeded a large stone, the gentleman being immediately called to the spot, directed a minute examination, which led to the discovery of an extensive grotto, completely studded with shells in curious devices, most elaborately worked up, extending an immense distance in serpentine walks, alcoves, and lanes, the whole forming one of the most curious and interesting sights that can possibly conceived, and must have been executed by torch light. We understand the proprietor intends shortly to open the whole for exhibition, at small charge for admission.
It has remained in private ownership ever 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 it 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.
References.
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