A few weeks ago, residents in the city of Foshan in China's Guangdong province were alarmed when a floating city suddenly appeared in the clouds above them. The phenomenon lasted only a few minutes and seemed to contain skyscrapers and buildings suspended mid-air. A few days later, a similar thing was reportedly seen in the province of Jiangxi. As you can imagine, people were pretty freaked out.

The news footage has since gone viral, and the Internet has been going nuts with theories. There's been talk of a gateway to a parallel universe and, worryingly, a lot of discussion about some bizarre NASA plot to simulate the second coming. But the reality is less conspiracy and more physics.

First of all, it can't be ruled out that the footage of the cloud city isn't digitally altered - there's only one video (below) circulating, which seems a little suspicious given that thousands of people reportedly saw the phenomenon. But assuming that it is real, what could be causing it?

The most reasonable explanation is something called Fata Morgana, a type of mirage that basically projects a distorted reflection of a real object. In this case, the city of Foshan's own skyline seems to have ended up being reflected into the clouds.

A Fata Morgana happens under a special set of conditions known as a thermal inversion, when warmer air sits on top of cooler air. Usually in weather systems, the opposite happens, which makes this type of mirage incredibly rare, and mostly seen near the coast.

In these conditions, because the air layers are different temperatures, they also have different densities, which means they refract - or bend - rays of light at different angles. So light being reflected off an object - say, an entire city skyline - will be bent before hitting your eye.

How does that create a mirage? Well, the rest is all down to our brains, as Matt Simon explains for Wired:

"When light hits your eyes, your brain assumes it arrived there in a straight path between you and the object reflecting the light. So if light is bent on its way toward you, your brain will think the object is where it would be if the light’s path was straight."

You can see it explained in the diagram below of a 'superior mirage':

Zátonyi Sándor/Wikimedia Commons

This wouldn't be the first time that a Fata Morgana has caused some confusion, either. In the past, the mirage has caused people to see castles in the air, floating boats, and double mountains.

But we have to admit that seeing an entire city appear out of nowhere in the sky above you would have to be slightly more unsettling. Still, next time you see something strange, let's all assume physics before conspiracy theory, okay?