From Great Big Story, this fantastic (and mysterious) little tidbit:
Less than six feet beneath a residential neighborhood in the English seaside town of Margate lies a mysterious underground cavern. Nicknamed Shell Grotto, the subterranean passages wind for 70 feet and are adorned by millions of seashells. Who built the cavern and for what purpose remain a mystery.
The relentlessly-reliable Wikipedia says that "the purpose of the structure is unknown, and various hypotheses have dated 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."
Personally, if this discovery does not involve Indiana Jones in some way, I'll consider it a huge missed opportunity.