Taos, New Mexico isn't the only place where a bizarre ambient hum is heard, but it's probably the most famous spot for the mysterious droning, so much that it's mentioned in travel guidebooks about the area. The Taos Hum reportedly is heard by only 2 percent of the local population and been described as a sound similar to the low rumble of a diesel truck's engine [source: Eisenberg].
People who noticed it are bothered to varying degrees, with some experiencing a mild feeling of irritation, while others report sleep disturbances, dizziness and nosebleeds. Conspiracy theorists suggest that it's caused by some sort of secret military communications system used to contact submarines, or a clandestine weapons testing program.
However, scientists think it may be caused by low frequency waves that originate in the atmosphere or else by vibrations from deep within Earth. A third hypothesis is that some people are extraordinarily sensitive to certain electromagnetic frequencies, and that the hum is caused by devices such as cell phones. But again, nobody really knows for sure [sources: Strange Sounds, Eisenberg].