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Uh-oh Moscow, who ya gonna call? Photographer Alexander Lukinsky took the incredible image above of strange, ethereal clouds hovering above the Russian capital on Tuesday night. But rather than signaling the impending arrival of Gozer the Gozerian, what Lukinsky caught on his camera was a relatively rare phenomenon known as noctilucent (‘night-shining’) clouds.

Noctilucent clouds are still surrounded by mystery. There are no recorded observations of them prior to 1885, suggesting they are a relatively new meteorological phenomenon, and possibly linked to global warming. What we do know is that they occur at incredible altitudes: at 76–85 kilometers (46–51 miles) high, they are above the stratosphere. This height helps give them their eerie appearance: the tiny ice crystals from which they are composed are hit by the sun from underneath – giving them a silvery-blue appearance – despite the sun being below the horizon line, meaning the viewer sees these shining clouds from a position of darkness.

While they are rarely sighted, it seems that noctilucent clouds are showing themselves a bit more lately – astrophotographer Christoph Malin captured the time-lapse footage below in London last week, and many other amateur sky-watchers have captured the clouds on camera as well.