The release highlights how the reported shapes of UFOs have changed during the past half-century, the National Archives said. Many of the reports in the latest file describe UFOs as big, black and triangular, whereas reports from the 1940s and '50s tended to be about saucers or disc-shaped objects, they said.
"In the 1950s the next big leap in technology was thought to be a round craft that took off vertically, and it's intriguing to note that this is the same period when people began to report seeing 'flying saucers' in the sky," said David Clarke, author of a book called The UFO Files and a journalism lecturer at Sheffield Hallam University.
He pointed out that in the years covered by the latest file release, triangle-shaped U.S. stealth bombers and Aurora spy planes featured heavily on TV shows like "The X-Files" and movies like Independence Day.
"It's impossible to prove a direct link between what people are reading and watching and what they report as UFOs, but one interpretation could be that the latest advances in technology may be influencing what people see in the sky," he said.
A craving for chocolate may have influenced another sighting, that of a "Toblerone-shaped" UFO hovering over Annandale, Scotland, in July 1994. The Toblerone is a triangular-shaped Swiss chocolate candy bar. The files include a sketch of the object, which showed it as 35-40 feet long and about 20 feet wide.
More here: Britain releases new UFO files - CNN.com