It was the city of Chicago, some 50 miles away.

Joshua Super and Shalee Blackmer were camping at Michigan’s Warren Dunes State Park when they noticed strange rectangular figures looming on the horizon of Lake Michigan.

The eerie effect was the product of a special atmospheric condition called a “superior mirage.” Superior mirages occur when a large area of warm air sits over a layer of colder air, a phenomenon called a temperature inversion. They’re actually common in the Arctic, where abnormal temperature gradients are more prevalent. As light rays pass through a temperature inversion, they are bent downward due to the higher density of the cold air, resulting in an inverted image that appears to float above its actual position.

Sometimes an object below the horizon may even become visible. In this case, the optical ray curvature was stronger than the curvature of Lake Michigan’s surface, which made it possible for Super and Blackmer to see the city’s buildings—even from across the Great Lake.

Super posted his snapshot of the Chicago superior mirage on Reddit, and it’s received nearly 1.5 millions views since.

Here’s Keith Matheny of the Detroit Free Press, interviewing Andrew T. Young, a San Diego State University astronomer and mirage expert: