There is a story about the Paris catacombs that I love dearly. In August of 2004, several police officers were exploring a section of the infamous maze of tunnels near the Eiffel Tower when they came across a particular doorway covered in plastic with a sign that said, "No entry."
Inside, the police were momentarily terrified by the sound of attacking guard dogs, but they realized it was a recording. Pushing further into the tunnel, they found a full working cinema, complete with lights, a projector, a bar, a dining area, and seats carved directly into the rocks.
When they went topside to report their find to their superior officers, they were pleased with what they'd found. By the time they got back, though, everything was gone, and all that was left was a note that said, "Do not look for us. Signed, The Society Of The Perforated Mexicans."