The purpose of the structure is unknown and various hypotheses date its construction to any time in the past 3,000 years. Hypotheses include: it was an 18th or 19th-century rich man’s folly; it was a prehistoric astronomical calendar; it is connected with the Knights Templar or Freemasonry. Since the 2007 discovery of a domed cave under the Palatine Hill in Rome with shells, mosaics and marble in similar patterns to those in the Rectangular Chamber in Margate, some credence has been given to the theory that the Shell Grotto could have been created by the Phoenicians in the second half of the first millennium BCE, when they were founding many colonies from their base in Carthage.