In Windsor, Ontario, a Canadian city just across the border from Detroit, a persistent but irregular hum has become more than just a baffling mystery. It’s frustrating residents, causing them to report declining quality of life and possibly damaging their health. People liken the sound to the idling of a diesel truck or a concert subwoofer, according to The New York Times.

Even experts can’t figure out where the sound is coming from. The Canadian government has studied it. Researchers from the University of Western Ontario and the University of Windsor have studied it. One report from the University of Windsor suggested that it might be coming from blast furnaces on Zug Island, located on the Detroit River, operated by United States Steel. But the company hasn’t been cooperative with activists hoping to get to the bottom of the issue (and ultimately stop it), and the noise hums on.

It’s not just a mass delusion or a conspiracy theory perpetuated by UFO obsessives. Not everyone in Windsor hears the hum, but thousands of people do. There’s a private Facebook page dedicated to discussing it, and people from Windsor call Tracey Ramsey, their Parliament representative, to complain of headaches, mental health issues, and insomnia caused by the noise.

But Windsor isn’t the only town with a mysterious noise problem. The World Hum Map and Database Project, run by a high school teacher in British Columbia, has been collecting and mapping reports of hums since 2012, showing that it truly is a global phenomenon. The persistent, unexplainable sound of a hum, much like the sound of a generator, has affected people across the world, from Scotland to Florida to South Africa. Some of these noises can be traced back to concrete sources, like ultra-low-frequency sound from factories, or in one case, the mating calls of a fish, but most, according to the New Republic’s 2016 investigation of the worldwide phenomena, remain mysteries even after research by universities and government officials. “It’s like chasing a ghost,” University of Windsor professor Colin Novak told the CBC of the study he led on the Windsor hum. The town of Windsor may never find out for sure what the sound is, or how to stop it.

[h/t The New York Times]