We live in a world saturated by noise. From the drone of passing traffic to the incessant "dinging" of our smartphones, we are surrounded by acoustic pollution. When researchers were dispatched to Taos, New Mexico, they tuned their acoustic measuring equipment to the lowest audible frequencies, between 8 and 80 Hz [source: Mullins and Kelly]. These are the ultra-bass frequencies that register more as a rumble and a throb than a perceptible tone. But investigators were unable to isolate a single environmental source that emitted tones at that frequency.
In Kokomo, acoustic consultants tracked down two possible culprits, a compressor station and an industrial fan. But when both machines were powered down, locals claimed that the Hum hummed on [source: Deming]. In 2014, researchers in Windsor, Ontario, may have isolated a local hum to the blast furnace of a steel plant on Zug Island in nearby Michigan. But as of early 2015, they needed to confirm this with U.S. Steel [source: Pearson].
However, local industrial sources don't explain the worldwide prevalence of the Hum. What else could produce such powerful and pervasive low-frequency tones? What about long-distance radio transmissions? There's a network of radio transmitters called LORAN (long range radio navigation) that broadcasts low-frequency signals as a form of primitive GPS. Skeptics point out that if LORAN was the culprit, then we would have more reports of the Hum closest to LORAN towers. Also, LORAN broadcasts 24/7, but Hum sufferers mostly complain of the noise at night [source: Deming].
A more intriguing possibility is TACAMO aircraft, military planes that employ radio frequencies in the lowest end of the spectrum to communicate with submerged submarines. The planes operate at night, and their movements are top secret. Hum hearers in Largs, Scotland have long believed that their particular hum originates from the local naval base [source: Barton]. The TACAMO theory might also explain why many Hum sites are on the coast. For example, residents in the tiny Massachusetts coastal towns of Nahant and Hull can hear the Hum, while it goes undetected in nearby Boston [source: Deming].
Conspiracy theorists have dreamed up all kinds of wild stories for the source of the Hum, including a large-scale mind-control scheme executed by the U.S. and U.K. governments. Others believe that regardless of its source, the hum is dangerous enough to drive people temporarily insane, and is a possible cause of mass shootings in the U.S. [source: Keller].
Of course, there's one more theory about the source of the Hum — it's all in our heads!