Shell Grotto, Margate

Some four and a half million shells spiral across the walls and ceiling of an underground passage that nobody can explain. The Shell Grotto in Margate was discovered in 1835 — the story goes that a boy fell through a hole in a field — and it has resisted every attempt at dating, attribution or rational explanation since.

The grotto is a winding passage, a rotunda and a rectangular chamber, every surface covered in shell mosaic of extraordinary complexity — cockles, whelks, mussels and oysters worked into floral runs, sun symbols and what may be religious iconography. Theories span two millennia: Phoenician temple, Roman shrine, Masonic chamber, Regency folly. Carbon dating has proved inconclusive, not least because Victorian gas lighting sooted the shells, and the not-knowing is now the point — every visitor leaves with a preferred theory and no proof. Above ground, Margate's old town has reinvented itself around the Turner Contemporary gallery, the retro delights of Dreamland and the tidal pools of the main sands, so the town bridges genuine mystery and seaside cheer inside a few streets.