NO ONE KNOWS WHEN THIS WAS BUILT, or when it was built, except that it is extremely old.
Thirty-five miles north of Dover is the English town of Margate in Kent - a coastal town with 57,000 inhabitants and a proud maritime history. What makes Margate special is the presence of a mysterious grotto, which has come to be known as the ‘Shell Grotto of Margate’. In 1835, the local school principal, James Newlove, wanted to build a duck pond in his garden. While digging, his shovel disappeared into an opening underneath a displaced capstone. He lowered his son Joshua on a rope to retrieve the item. Upon returning the boy spoke of tunnels full of shell decorations. Excavations revealed a spectacular grotto covered by more than 4.6 million seashells, spread in a mosaic over 600 m2. Since the discovery in 1835, people have speculated about the true meaning of this place. What makes the shell grotto of Margate so mysterious is that there is nothing known about it.
Source: Ancient Origins