The Hampton Court Palace CCTV ghost ("Skeletor"), 2003

Hampton Court Palace stands on the River Thames, 10 miles west of central London. The Associated Press called it "the Tudor castle outside London built in 1525." It has a long reputation for ghosts. Jane Seymour is said to walk Clock Court in white, carrying a candle. Catherine Howard belongs to the Haunted Gallery.

In 2000 the psychologist Richard Wiseman led 1,027 volunteers through parts of the palace. More unusual experiences were reported in the areas said to be haunted, and they tracked variations in the local magnetic field and in lighting.

October 2003. A fire door near Clock Court flew open. Then it happened again, three nights in a row. Some accounts say days. Palace security went to the CCTV to find out who kept leaving it open.

On the second occasion, the camera caught someone. A figure in a long, robelike garment, in period dress, stepped from the shadowy doorway. One arm reached out for the door handle. The area around him was somewhat blurred. His face appeared unnaturally white compared with his outstretched hand.

Security guard James Faukes put it simply. "It was incredibly spooky because the face just didn't look human. My first reaction was that someone was having a laugh, so I asked my colleagues to take a look." Staff asked the palace's costumed guides. They did not own a costume like that. Around the same time, a visitor wrote in the visitor book that she had seen a ghost in the same area.

19 December 2003. The palace released a still from the footage. AP, CNN and NBC ran it that day, and the BBC the next. The Telegraph, the Sun and the Daily Mail carried it too. Palace spokeswoman Vikki Wood was asked whether it was a Christmas hoax. Her answer was firm: "it's not a joke, we haven't manufactured it." Then: "We genuinely don't know who it is or what it is." The palace asked the public to come forward with explanations.

Sceptics moved fast. On the Straight Dope message board, one poster wrote: "It's a guy in a robe with a white mask. 25.95 from your local costume hire." Another suspected "a staff member fooling around, then this being picked up and promoted by the publicity officer." Others noted that October means Halloween costumes, and that two months had passed before a pre-Christmas release. Some simply asked why a ghost would need to open a door.

The blogger Matt Jones answered with a satirical recipe, "How to Make a Hampton Court Ghost." Its ingredients included "A large, mysterious wooden door (opened)" and "A willing employee (in period costume)".

Against all that stood the guides, who had no such costume, and the palace's denial. No police, clerical or scientific investigation of the footage is on record. Even the retellings disagree. Most put the figure on the second night, one on the third. Some say he opened the door. Others say he shut it.

The figure was never identified. "The culprit has never been found," as one later account put it. Staff named him after the villain in He-Man and the Masters of the Universe, and they still call him Skeletor. The palace has had other camera ghosts since. In 2015, 12-year-old Holly Hampsheir photographed a cloaked figure in the King's Apartments. Metabunk explained that one as an iPhone panorama glitch, with a costumed guide walking through the shot. The 2003 footage has never been explained.