A cloud formation in China has baffled thousands and ignited conspiracy theories after appearing to resemble a 'floating alien city'.

The heavenly formation appeared in two separate incidents, first early last week in the city of Foshan, and a few days later in the province of Jiangxi.

The collection of clouds were said to look like a landscape of towering skyscrapers, with conspiracy theorists speculating it could be from an opening into a parallel universe, or be evidence of a secret NASA project called 'Project Blue Beam'.

Project Blue Beam is a conspiracy theory which claims NASA will someday simulate an alien invasion of Earth or second coming of Christ through holograms.

"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," YouTube channel Paranormal Crucible posted on YouTube.