Discovered in 1835 and still not satisfactorily explained, the Shell Grotto is one of England's most genuinely mysterious places. Every surface of the underground passageway is covered in intricate shell mosaics, over four million shells, and nobody knows who made it, when, or why.
It's small, strange, and completely unlike anything else. Entry is cheap and the experience stays with you. Exactly the kind of thing Margate does better than anywhere.