No, this isn’t the plot of Bioshock Infinite come to life. IFLScience brings us word that thousands of people in China this week reported seeing a large “floating city” in the sky that actually turned out to be a Fata Morgana, which is a rare type of mirage.

FROM EARLIER: In pictures: iOS 9 vs. Android 6.0 Marshmallow user interfaces

As you can see in this news video, this particular mirage is absolutely massive and really does look like a giant city that’s floating in the clouds.

But how is this possible? This page from San Diego State University offers a nice introduction to mirages and explains how giant mirages such as this one are formed.

“[Mirages] are real phenomena of atmospheric optics, caused by strong ray-bending in layers with steep thermal gradients,” the page explains. “Because mirages are real physical phenomena, they can be photographed.”

Essentially these mirages occur whenever the sun heats up a layer of the atmosphere even as the layer below it remains at a cooler temperature. This causes light travelling through the atmosphere to get refracted and creates the illusion of objects in the sky like the giant city we see in the video.

For a much more detailed explanation of how these kinds of mirages work, we recommend checking out San Diego State University’s full page on them here.