Shell Grotto, Margate

The Shell Grotto is an ornate subterranean passageway shell grotto in Margate, Kent. Almost all the surface area of the walls and roof is covered in mosaics created entirely of seashells, totalling about 190sq metres of mosaic, or 4.6 million shells. It was discovered in 1835 but its age and purpose remain unknown. The Grotto is a Grade I listed building and is open to the public.

The Shell Grotto consists of a winding subterranean passageway, about 2.4 metres high and 21 metres in length, terminating in a rectangular room, referred to as The Altar Chamber and measuring approximately 5 x 6 metres.

The excavations are entirely underground. Steps at the upper end lead into a passage about 1.07 metres 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.

Attached to the grotto is a museum, gift shop, and cafe.

The purpose of the structure is unknown, and various hypotheses have dated 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/Freemasonry. No scientific dating of the site has been done.

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 would have been collected from shores west of Southampton, where it is abundant.

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