The earliest reference to the discovery appears in an 1838 article in a predecessor of the Kentish Mercury, and most surviving accounts agree on a date of around 1835. The standard story has a workman digging in a Margate field stumbling onto a hole, lowering a child on a rope to investigate, and the child returning with breathless tales of shells everywhere. The truth is messier - several conflicting versions circulated even in the 1830s - but the grotto was first opened to the public as a private tourist attraction in 1837. It was listed Grade I in due course, which is the highest level of heritage protection England offers. The owners have charged admission for nearly two centuries. The shells survive remarkably well in the underground micro-climate, though Victorian gaslighting did damage some areas through soot deposition, and conservators have spent decades doing painstaking restoration of damaged sections. The dome at the centre has a shaft to the surface, capped to let in muted daylight - which means that even in deep winter, the grotto is gently lit during the day.

The grotto consists of a 70-foot serpentine passage roughly 3 feet wide, leading down via steps from the surface, eventually reaching an arched threshold where the shell decoration begins. The passage continues, curving in mosaicked vaults, until it opens into a central rotunda topped by the dome. Three arches lead from the rotunda - two back along the serpentine passage, one continuing toward the rectangular 'altar chamber' that terminates the grotto at the far end. The altar chamber's decoration is more formal and geometric than the flowing patterns of the passage: star and sun shapes, framed niches, a curved focal alcove that gives the chamber its modern nickname. Mussels, cockles, whelks, limpets, scallops, and oysters dominate. Most of these are local - gatherable in sufficient numbers from Walpole Bay in Cliftonville, Pegwell Bay (especially at Shellness Point near Richborough), Sandwich Bay, or Shellness on the Isle of Sheppey. The flat winkle, which fills the backgrounds between the figured designs, is found only rarely on the Margate coast - and was probably collected from shores west of Southampton, where it is abundant. Someone carried shells, in serious quantity, from at least 150 miles away.

The grotto invites speculation, and a century and a half of writers have happily speculated. A late-1940s book argued for Phoenician origins - and was written by an author also enthusiastic about the Baconian theory of Shakespeare authorship, which is not a recommendation. In 1952 a member of the Canadian Geographical Society claimed similarities between the shell patterns and Bronze Age Minoan art. Other suggestions have invoked Templar mysticism, Masonic ritual, druids, and the inevitable lost civilisation hypotheses that attach themselves to any unexplained subterranean space. The most measured assessment came from a member of the Kent Archaeological Society in 2006, who concluded that the grotto was likely a medieval denehole - one of the small chalk mines common in this part of Kent - that was reworked and decorated in the 17th or 18th century, with the decoration possibly extended in the early 19th. This is sober. It is probably mostly right. It also leaves entirely unexplained why anyone would haul winkle shells from Hampshire to ornament an abandoned chalk mine in Margate, and what the geometric altar chamber was actually for.

Modern dating techniques have been tried and failed to settle the question. Carbon dating of the lime mortar holding the shells produced contradictory results. Stylistic analysis of the patterns has been inconclusive - the designs are too generic to firmly date, too elaborate to be casually executed, too well-preserved to be ancient by Mediterranean standards but too elaborate to be straightforwardly Georgian. The New York Times Magazine ran a piece in October 2024 titled 'The Mystery That's Too Good to Solve' - acknowledging that there is something almost protective in the grotto's status as an enigma. Researchers occasionally argue for new theories. Conservators worry about damp. Visitors descend the steps into the cool chalk passage and walk through the shells, taking photographs they will struggle to explain, asking the guides questions the guides cannot fully answer. The grotto's age, creators, and purpose are unknown - the official Wikipedia description, repeated word for word for years - and that admission may be the most honest thing English heritage has ever written about any building it owns.