Beneath an ordinary English town lies one of history’s strangest mysteries. In 1835, a man in Margate discovered a hidden underground chamber beneath his backyard and what he found was unbelievable. A network of tunnels and rooms, every single surface covered in millions of seashells. This place is known today as the Shell Grotto. But here’s the mystery: no one knows who built it, when it was built, or why. Some believe it was an 18th-century folly, while others think it could be centuries older