As one story goes, in 1835, James Newlove sent his son Joshua down a dirt hole that appeared suddenly while he was digging a duck pond. Newlove was the schoolmaster of a local boys’ school in Margate, Kent in the U.K., and when Joshua emerged with a tale of vast secret passageways lined with intricate seashell mosaics, Newlove’s life and career changed forever.

In the process, the world also gained a mystifying treasure.

Father and son had discovered the Shell Grotto lying quietly beneath an ordinary farm field — a gorgeous mystery that remains unsolved today. No one knows who cut through Kent’s underground chalk deposits to create this astonishing subterranean cavern with its 70 feet of winding walkways leading to a large chamber. Nor is it clear how they transported some 4.6 million shells and painstakingly adorned nearly every surface — the walls and arched ceilings, all 2,000 square feet of them — with exquisite, enigmatic symbols and designs.

Was it an ancient temple used for pagan rituals? A meeting chamber for a secret cult? Or even a stylish man cave for some seashell-loving nobleman?

shell grotto closeup Close-up of a mosaic. Almost every surface within Shell Grotto’s underground passageways is covered with ornate designs created entirely of seashells. (Photo: Emőke Dénes/Wikimedia Commons)

A deliciously unanswered question

Since Newlove opened the Shell Grotto to visitors in 1838, there have been no lack of theories, but definitive answers have remained stubbornly out of reach.

Located 90 minutes by train from London on England’s southeast coast, the Shell Grotto continues to be a popular tourist attraction in the old seaside town of Margate. The farm field that once overlaid the Grotto has given way to city streets. You’d never guess a cavernous network of decorated passageways lies beneath this ordinary-looking residential neighborhood in the heart of town.

shell grotto neighborhood Overhead view of the Shell Grotto museum shop (center) located in a quiet Margate residential neighborhood. Beneath it lies the magnificent mosaic-covered passageways. (Photo: Google Maps)

The Grotto is now owned and operated by Sarah Vickery, who fell in love with it during regular visits as a child and decided to buy when it came up for sale in 2001. Built in a serpentine shape, visitors follow a curving entrance passage to a rotunda with a dome that opens to the sky. From there they follow an S-shaped serpentine passage, which leads to a large rectangular altar room. Most of the shells lining the walls (mainly mussels, whelks, oysters, cockles, limpits and razor shells) are native to the English coast and were likely collected nearby.

closeup shell grotto The estimated 4.6 million shells that adorn the Grotto were probably collected locally near the seaside town of Margate. (Photo: Martin Hearn/flickr)

The Grotto has suffered some water damage over the years, and one wall was marred by a blast by German bombers in 1940. The shells are also darkened with carbon deposits from gas lamps that were used before the Grotto was electrified in the early 1930s.

Despite all this, the array of abstract suns, stars, trees, hearts and flowers that fill its spaces remain mostly undiminished and breathtaking. It’s their meaning that remains a puzzle, mainly because the symbols don’t conform to a single culture or time period.

shell grotto passageway Made from millions of seashells, the mosaic designs remain a mystery because they aren’t specific to one culture or time period. (Photo: Ben Sutherland/flickr)

What was it used for?

Over the years, researchers have come up with an intriguing list of possible uses for the Grotto (which would also help decipher the mosaic designs). Among them: a prehistoric worship space, an ancient Roman or Phoenician temple constructed for pagan rituals, a smuggler’s cave, and even an amusing "folly" hatched by an unknown 18th century aristocrat. Some, but not all, of these theories have since been debunked.

A more recent explanation comes from Mick Twyman of the Margate Historical Society, who concluded in 2006 that the Grotto was constructed by Knights Templar in the 12th century, and that the altar chamber was a temple for Masonic rituals. His research suggests the open dome located in the rotunda may have been built to function as a solar calendar.

shell grotto dome One theory suggests the open dome in the rotunda may have functioned as a solar calendar. (Photo: Martin Hearn/flickr)

Another plausible theory from the Kent Archaeological Society maintains that the Grotto is actually a medieval chalk mine, which was expanded and decorated in the 17th or 18th century for some undetermined purpose.

Which brings us back to who, why and how, as the BBC video below explores.

The riddle of this stunning hidden gem seems destined to remain unsolved.o.