YouTube The "City in the sky" and (inset) close up

Onlookers, some who are said to have videoed the bizarre event, were said to be mesmerised as a towering city of sky scrapers appeared from the clouds. First thousands reportedly saw a ghostly alien city floating over Foshan in the Guangdong province of China. A few days later people in the province of Jiangxi, China, also reported seeing a similar cloud city.

YOUTUBE Thousands of people were said to be stunned by the apparition

Weather experts have explained the phenomena as natural mirage, an optical illusion called Fata Morgana. Fata Morgana can be seen on land or sea and involves the optical distortion and inversion of distant objects such as boats, which can appears as skyscrapers because the images become stacked, when rays of light bend as they pass through air of different temperatures such as in a heat haze. But the mystery has sent conspiracy theorists into over drive.

ITN•YouTube A similar, if not more detailed, phantom city seen over China in 2011

It is also possible, considering China's technological achievements that a top secret holographic technology was tested over a heavily populated area in an effort to gauge the public reaction. Paranormal Crucible video

There were previous reports of a similar sighting in China in 2011. YouTube channel Paranormal Crucible said in a video report: "The footage captured by a local resident appears to show a huge city floating in the clouds. "The apparition, which was witnessed by hundreds of shocked residents, only lasted a few minutes before completely disappearing." The channel speculated it could be "the result of a project blue beam test." Project Blue Beam is a prolific conspiracy theory which believes NASA will one day simulate an alien invasion of Earth or second coming of Christ through holograms.

YouTube•CTI The story was so strange in made Chinese TV news channels

YouTube•CTI Witnesses tell TV reporter of the sighting

The Truth Is Out There: 150 years of UFOs Mon, July 3, 2017 Although many UFO sightings often have rational explanations, a small number truly are unexplained. Here's our pick of the most compelling UFO sightings from all over the world Play slideshow Getty Images 1 of 27 Flying saucer above highway, real or not?