The Shell Grotto on Grotto Hill, Margate, is unique in the British Isles. It is a man-made cave system whose walls are entirely covered with shells. It lies North of Margate's town centre, near the Northdown Road shopping area.

The Shell Grotto was discovered in 1835 by two schoolboys who found a hole in the ground, into which one of them fell, to find a dazzlingly colourful cave. Unfortunately, today the cave no longer retains its former vividness, having been flash-photographed many times by barbarian tourists over its life. The ownership have now, quite sensibly, banned flash photography, instead selling colourful postcards.

The grotto has a small reception area at the front, resembling an old shop, where one can buy the aforementioned postcards, souvenirs, and the all-important guidebook. The cave system itself is entered by a door at the back of this shop area, after paying the fairly reasonable price of £1.50 for adults and 75p for children under 12. Groups get a 10% discount. At the bottom of the few steps, fluorescent lights illuminate the shells. Take a good look at them.

The attraction's tourist literature tells us that it is more than a thousand years old, and of Phoenician origin. Looking at the shells, however, reveals pagan imagery alongside Indian symbols, confusing us more to the cave's age. As with many other mysterious attractions, there have been many theories put forward as to its original purpose, although the one I like best - and the most plausible - is that it was used as a pagan temple. One need only look at the fertility symbols to find evidence for this.

This, however, should not be the motivation for visiting the grotto. As you walk around it, an eerie spirit descends, and one is enthralled by the mystery and splendour of this forgotten cave. Forgotten for so long until its rediscovery, and unfortunately forgotten today, due to its location and lack of advertising. I, too, am guilty of forgetting it, having only visited twice in the past few years - the last time being last year. Were it slightly closer to home, I would be inclined to visit it for peace and inspiration - such is the silence and mystery of the place. Not that I would be the only one to have done this. C E Mitchell wrote The Story of the Margate Grotto as an investigative work spurred on by the mystery of its origin.

The Shell Grotto is not the only cave complex in Margate. In the same area, further inland, lie the Margate Caves, a huge complex which are not artificial and were used as recently as the turn of the century as smugglers' hideouts. They lack, however, the intrigue of the grotto, and fail to hold the attention for as long.

The Shell Grotto is open only in the high season (May to October), but is barely ever crowded during that time. Parking is a problem unless you don't mind a short walk up the hill from the town centre, where many pay and display car parks are provided at reasonable prices.