If there's one thing Prince Philip enjoys, which might be out of the ordinary for a royal, it's his growing collection of books about aliens, UFOs, and outer space. The Duke of Edinburgh is reportedly an avid reader of this genre, and he likely picked this up from Lord Louis Mountbatten, his uncle.

According to reports, Lord Mountbatten kept a secret account of a UFO that allegedly landed in Prince Philip's family estate. A bricklayer by the name of Fred Briggs saw the flying saucer while he was working on the Broadlands in 1955.

The unknown spacecraft had an occupant, whom Briggs described as a figure dressed in overalls and a helmet. He encountered the figure in Mountbatten's estate, but his account wasn't made public until Lord Mountbatten died in 1979.

Daily Star reported that Prince Philip has been probing aliens since then and he has a map of UFO sightings on one wall in his former office at Buckingham Palace. A senior Palace aide said that the Duke of Edinburgh was always curious about "close encounters" reports and would ask for these top-secret documents.

In the biography Prince Philip: The Turbulent Early Life of the Man Who Married Queen Elizabeth II, author Philip Eade shared that Queen Elizabeth's husband would invite "witnesses" at Buckingham Palace to talk about their encounters with aliens. His equerry Sir Peter Horsley said that the Duke of Edinburgh would apparently weed out the truth from these witnesses.

Prince Philip also subscribed to Flying Saucer Review and once asked Horsley to go to a house in Ealing, where an alien allegedly lived. The Duke of Edinburgh also acquired a book called Haunted Skies: The Encyclopedia of British UFOs.

Last year, Prince Philip read The Halt Perspective in one sitting. The book is about a sighting at the Rendlesham Forest involving U.S. Air Force officials who were there to investigate a 1980 plane crash. Instead, the officials saw a different aircraft with hieroglyphic symbols.

Another author in this genre, David Clarke, said that Prince Philip had been looking forward to meeting the American George Adamski in person. He is apparently the "first contactee" who claimed that aliens from the planet Venus abducted him and took him around the Solar System.

Buckingham Palace, however, never pushed through with Prince Philip's meeting with Adamski, over fears of embarrassment, sources said. Years later, Adamski's claims were investigated and proven as a hoax.