Altair tried to find the best observation spots by tapping the Light Pollution Map created by Jurij Stare, with data sourced from NASA’s Visible Infrared Imaging Radiometer Suite (VIIRS) and the NOAA’s Defense Meteorological Satellite Program (DMSP). That led him to wonder where a lot of stray light pollution was coming from in spots with no signs of urban development.

“Of course, the vast majority of light pollution is from cities: street lights, industrial zones, parking lots,” he wrote. “But sometimes, it’s from something else.” To start with, he searched his home state of Maine and found an enormous light pollution source with seemingly nothing around it. After using Google maps to zoom in, he spotted a massive hothouse complex. “This greenhouse augments its growing season with an enormous array of yellow lamps,” he wrote. “And this makes it the single greatest source of light pollution in the entire state.”

He also spotted what looks on the map like one of the largest sources of light pollution in America, smack in the middle of… North Dakota? After tracking down the blotchy feature on Google Maps with a satellite overlay, all he found at first was “an intense lack of urbanization.”