Air traffic controllers have been left stumped after a member of public photographed a strange plane contrail over Hampshire.
The shape of the trail looks to turn one way before quickly turning back again.
Teacher Florence Lehmann, spotted the trail as she left her home in Southsea at 7.20am on Monday. She told the Mirror: "It's really weird. It looks like the pilot sneezed or fell asleep.
"Somebody said a UFO had been spotted recently in the area and the pilot had tried to avoid it, which I quite like the sound of.
"There was no wind this morning and the plane actually moved trajectory."
According to Sky News, a spokeswoman for Nats, the air traffic control company, said she could not explain why the plane changed direction, saying: "Unfortunately Nats has no knowledge of the purpose of any given flight and we do not identify individual aircraft to members of the public."
One local even described it as something he could now tick off his bucket list.
Bill Cooke, head of NASA's Meteoroid Environment Office, originally confirmed the footage as a bright meteor - but then said he believes it was a plane's contrail, or vapour trail.
According to the Daily Mail, he told Life's Little Mysteries: "Reviewing the video, it does appear to be a contrail."
10 things we've mistaken for UFOs
Mystery as plane leaves strange vapour trail in sky
This UFO-shaped cloud caused a stir in China when it was spotted hovering in the sky above Jilin Province. The 'lenticular cloud' was pictured moving fast near the Changbai Mountain at the boundary of China and North Korea. After two minutes it was absorbed into a huge cloud cluster and disappeared.
Yes, you read that right. In 2011, a man called 999 to report an unidentified flying object above his house in Hertfordshire. He sounded genuinely panicked and told call handlers blazing lights were coming towards him. Two minutes later the man called back when he realised what he saw was the Moon!
A Swedish diving team found a strange object with "rounded sides and rugged edges" at the bottom of the Baltic Sea in 2011. Some believed that a UFO had crashed into the sea, while others speculated it could be the remains of a World War II German anti-submarine device. Most experts concluded it was most a natural geological formation rather than an "anomalous" object.
This picture shows what looks like the incredible moment a 'UFO' hovered over Seattle. It was in fact the unusual cloud formations around the iconic Space Needle appearing to reveal an alien ship in the sky above the city. But thanks to the building's saucer-like shape it made for a convincing UFO snap.
A rare occurence in 2008 saw Venus, Jupiter and a crescent moon appear together in the night sky from Portsmouth. While planets have been mistaken for UFOs, experts say the sighting was a 'conjunction'. Robert Massey of the Royal Astronomical Society told the Daily Telegraph that "they just appear close in the sky". He added: "I wouldn't be surprised at all if the... police and the [Ministry of Defense] and the [Royal Air Force] get calls about UFO sightings. Because it's a spectacular sight and quite unusual and I think for that reason people really will be struck by it."
Weather balloons are often mistaken for unidentified objects but when they explode they can really look like something that occurs deep in space. When a man from California was scanning the night sky and saw a sudden burst of light through his telescope he was convinced he'd seen a UFO. "I saw another object orbiting this - whatever it was up there, and I’ve never seen anything like that before," Elijah Prychodzko told CBS13. He sent it to a scientist who concluded it was a weather balloon exploding. It was explained as decreasing air pressure causing the rising balloon to expand until it bursts.
Light bouncing off our camera lenses can cause a flare that shows an otherworldly object in our photos and if well placed they can look like alien spacecrafts, like this lens flare that could be a spaceship following a car.
A series of bizarre lights were spotted shining in the sky over Cincinnati in 2012. They were compared to the 1997 Phoenix Lights and after various sightings it was soon revealed that they were not space invaders, but just a group of five skydivers wearing illuminated clothing and performing a pyrotechnics show at a high school event.
These unmanned objects are often deliberately created to look like UFOs to fool people, such as when pizza company Domino's' launched the remote-controlled 'Domicopter'. With more drones in the sky, there have been more UFO reports too. A flying object over a Canadian baseball game in 2013 turned out to be part of a marketing campaign from the H.R. MacMillan Space Center and a camera-mounted drone observing a large crowd of protestors in Moscow was identified as police surveillance. Military drones have also been mistaken for mysterious flying machines - residents in Washington D.C. once thought they had seen UFOs travelling along the Capital Beltway.
One of the most mysterious natural phenomena is ball lightning, which is seen as a flash during a storm and is difficult to capture on camera. Scientists have said ball lightning explains many UFO sightings, like when a green object was seen to roll over the mountains over Brisbane, Australia. Astrophysicist Stephen Hughes told the BBC: "If you put together inexplicable atmospheric phenomena, maybe of an electrical nature, with human psychology and the desire to see something - that could explain a lot of these UFO sightings."