Chicago was floating on Lake Michigan this week. It could be seen from 50 miles away on the opposite shore. In New Buffalo, Mich., and Michigan City, Ind., the skyline towered above the lake like it was just a few miles away — not 50.

It was a fata morgana, which sounds like something out of a necromancer’s spell book, but there’s an explanation as to why it happens that isn’t rooted in witchcraft (which people obviously pointed to when these things appeared and freaked people out few centuries ago).

And boy did it freak people out. In 1642, looking toward the Mediterranean on the island of Sicily, Father Domenico Giardina, a Jesuit priest, wrote that he saw “a city all floating in the air, and so measureless and so splendid, so adorned with magnificent buildings, all of which was found on a base of a luminous crystal, never beheld before.”

Then the image changed into forests and armies and a whole bunch of other things, according to Giardina, which must have been frightening at the time.

So the Jesuit priests started to ponder. They knew these false images had something to do with temperature and rising air, but they couldn’t quite put their finger on it.

They were onto something, though. The fata morgana, known technically as the superior mirage, has everything to do with temperature and the density of air. It’s the exact opposite of the mirage you see in the desert, beckoning you to an oasis that doesn’t exist, or the wavy blurred lines on a hot road on a summer day.

The fata morgana is only possible when there’s cold air near the ground and warm air above it. In other words, it’s a temperature inversion. Usually the temperature cools as you go up in the atmosphere — an inversion is flipped upside down. Instead of seeing the sky reflected in the ground, you see the ground reflected in the sky.

Doubly interesting: There’s obviously a part of the atmosphere above the superior mirage where there is cold air above warm air, since in the top photo you can see the tops of the buildings hovering above, upside down.

Thanks to Joshua Nowicki for sharing his fata morgana photos!