There are mysteries and there are mysteries, and one of the most intriguing sits at the back of a perfectly normal souvenir shop on a perfectly normal street, ten minutes’ walk uphill from the perfectly normal resort of Margate, at the far end of the north Kent coast.

At the back of the shop is a rock tunnel lit with small, flared ceiling lamps, the sort of the thing you find in old railway stations. They throw parchment light onto walls carpeted floor to ceiling with shells. Big shells, little shells, shells in circles, hearts and fans, shells smooth-side out and shells inside-out, shells of winkles, cockles, mussels, scallops and oysters.

That’s not the mystery, though. The mystery is that although the tunnel descends into the hill for almost 100 feet, dividing briefly to form an elegant rotunda and ending in a ‘celestial’ or ‘altar’ chamber, and although its 4.6 million shells are skilfully fixed to the walls in strikingly coherent patterns, nobody knows who built it. Or when. Or why.

There are shells of winkles, cockles, mussels, scallops and oysters, most found locally Credit: CHRISTOPHER PLEDGER

"It drives people nuts," said Sarah Vickery, who used to come here as a child and bought the Margate Shell Grotto when it came up for sale in 2001, "They insist that we must know, there must be documentation. But there isn’t. We have no idea of its history before it was found. Its discovery seems to have come as a great surprise to people in Margate."

Nobody knew it was there until a family called Newlove leased the land in the 1830s and the children found the tunnel, alerting the adults by appearing covered in chalk. Or did the digging of a new duck pond open up a hole? Whichever, it was open to the public by 1838.

It was discovered in the 1830s and soon opened to the public Credit: CHRISTOPHER PLEDGER

The grotto may date from the eighteenth century, when they were built as architectural novelties in the grounds of houses such as Stowe, Woburn Abbey, Pontypool Park and many others. The poet Alexander Pope built one in the 1740s under his modest Twickenham villa, starting with shells but later replacing them with early geological specimens.

However, as Sarah pointed out, not only was this farmland then, with no private estate that might have indulged in a folly, the shells themselves are unusual in that they are native.

"Back then the fashion was for exotics," she said, "to show the travelling you’d done." But these are homegrown, from Margate (where there are still wonderful shells), Walpole Bay, Pegwell Bay, Sandwich Bay or Richborough. The winkles alone are from the West Country.

The grotto was restored in 2007 Credit: CHRISTOPHER PLEDGER

As you descend, florid designs of hearts, flowers and sinuous curves give way to a more geometric style in the rectangular final chamber. Where I leapt out of my skin, because of... well, go and see for yourself. This room has some queen conches, the lone exotics.

Although today the shells are faded from damp and Victorian gas lighting, originally they would have been as gently coloured as a hand-tinted print, with the sandy yellows of the winkles, pinky-white cockles, creamy plates of oysters and petrol-blue insides of mussels.

What’s really fun about the Grade I-listed grotto, quite aside from the quality of the designs, intact and in great nick after an excellent restoration in 2007, and the staggering number of shells, is choosing your own interpretation of the mystery creator’s intentions.

Could these be womb shapes? Phalli? ("Let’s face it, they could be mushrooms.") Are we in a birth canal? A tunnel of Life and Death? A temple of pagan devotions? A private shrine?

"It’s a bit like looking at clouds" added Sarah, who’s heard it all before, "You see in it what you will. There’s the folly crowd, who see pattern, and the devotional crowd, who see symbolism. There are Phoenicians versus Templars. The one thing we try and do is stay neutral; instead of telling people what we think it is, we let them decide for themselves."

The Shell Grotto Margate, Grotto Hill, Margate CT9 2BU, 01843 220008; opens daily 10am to 5pm from Friday March 30 to October 28 2018 and then reverts to winter hours. Adults £4, concessions £3.50, children aged 4 to 16 £1.50, families (two adults and two children) £8, under-3s free.

Three more grottos to visit

Skipton Castle, North Yorkshire, 01756 792442; skiptoncastle.co.uk – the Shell Room occupies the east tower of the gatehouse and is extremely old, built 1625-29 by a French craftsman who used Guernsey shells, volcanic rock and coral from Jamaica to represent the four elements.

Pope’s Grotto, Twickenham, Middlesex; popesgrotto.org.uk - though much faded from its former self, the tunnel and grotto running under Pope’s Villa (long since rebuilt as a school) is fascinating and lovingly maintained by a charitable trust. It will open on April 12, May 19 and on June 9, 16 and 23 as part of the Twickenham Festival. Adults £6, concessions £5, under 12s free.

William Kent’s Grotto, Stowe, Buckinghamshire; nationaltrust.org.uk/stowe – a fine example of a grotto set into a self-conscious landscape, it was designed in the rococo style in the 1730s by the brilliant William Kent as part of his ‘Elysian Fields’. It has undergone many changes and was used as an eighteenth-century picnic venue. Gift Aid Adults £13.20, under 17s £6.60, families £33, under 5s free.