Thousands of people reported to have seen a mysterious floating city above China's skies prompting conspiracy theorists to look into the phenomenon and come up with several speculations ranging from parallel universes opening up to secret government tests.

The mysterious city was first seen floating above Foshan in China's Guangdong province on Oct. 7, with some residents able to capture footage of the phenomenon. Days later, the ghostly city was spotted by people in Jiangxi.

"The footage captured by a local resident appears to show a huge city floating in the clouds," reported Paranormal Crucible. "The apparition, which was witnessed by hundreds of shocked residents, only lasted a few minutes before completely disappearing."

Several theories have emerged to explain the sighting including the so-called Blue Beam Project, a popular conspiracy theory that claims NASA will attempt to use holographic image projection technology to simulate alien invasion or the second coming of Christ. The apparitions were said to test the technology and see how people would react.

Many of those who saw the apparition also believe they saw a parallel universe. Paranormal Crucible said that the phenomenon could have possibly been a "parallel universe material that briefly went into our own reality."

While the idea of the existence of a parallel universe is often considered as far-fetched, some scientists have already started looking into its possibility.

In a 2014 study, Michael Hall, from Griffith University, and colleagues suggested of a quantum theory based on the idea of interaction between parallel universes. The research team proposed that parallel universes interact and influence nearby worlds by a subtle force of repulsion.

It appears though that the cloud formation that the people in China have seen is not at all evidence of a parallel universe.

Weather experts said that the phenomenon could be attributed to an optical illusion known as Fata Morgana, a kind of mirage that distorts distant objects due to the sun heating up the atmosphere above land or sea creating a gradient of temperatures. The mirage typically appears as a city of skyscrapers due to successive stacked images.

It isn't also the first time that a ghostly city has been seen hovering in the clouds above China. Sightings of a colossal structure that resembles a city with skyscrapers floating above Huangshan City have also been reported in 2011.