People in cities around the world—from Taos, New Mexico, to Windsor, Ontario, to Blackburn, England, to Auckland, New Zealand—have reported hearing strange humming noises that have no obvious source. The hums are often compared to the sound of idling trucks, and for the people who can hear them, they can be maddening, causing nausea and insomnia.

City officials and scientists have investigated various potential causes of the hums, including industrial plants, electricity pylons, mating fish, and even mass hysteria. In the first episode of our new series “Sound Mysteries,” we look into the case of the mysterious hums that have vexed people across the globe.