In the early 1990s, residents of Taos, New Mexico, complained of a low humming or droning sound that others around them could not hear. A study at the time found that at least 2 percent of people could hear it, each at a slightly different pitch between 32 and 80 hertz, pulsing slowly.
Taos is one of several places with a well-known 'hum'. Others include Bristol in England, Auckland in New Zealand and Windsor, Ontario, where a hum was eventually traced to industry on Zug Island across the Detroit River. Researchers looking at Taos also considered sounds made inside the ear itself, which many people have without noticing.
The Taos Hum has featured on Unsolved Mysteries and in The X-Files.