A 19th-Century Mystery: A 70-Foot Tunnel Covered in 4.6 Million Shells

In 1835, a man digging a duck pond accidentally uncovered a 21-meter tunnel completely lined with 4.6 million shells. Located in Margate, UK, the 'Shell Grotto' remains an unsolved archaeological puzzle. No one knows who built it or when, leaving historians baffled and visitors intrigued by this unique structure.