In the 1990s, residents of Taos, N.M., and Kokomo, Ind., began to report an invasive low-frequency noise, which they likened to a distant idling diesel engine. Others have since reported the sensation elsewhere, especially in Europe. Strangely, the sound is often worse indoors, and ordinary microphones don’t detect it.

Possible explanations have included everything from meteors to submarines, but so far there’s been no large-scale investigation. For now the phenomenon is simply called “the Hum.”