Since acoustic researchers have failed to find a single measurable environmental source for this worldwide phenomenon, it's fair to ask: Is the Hum some kind of mass delusion?
Not likely, says David Deming, a geosciences professor at the University of Oklahoma who wrote a comprehensive paper on the Hum in 2004. In previous cases of mass delusion — like the witch hunts that erupted across Europe in the 15th and 16th centuries — the perpetrators had something to gain by joining the mad throngs, namely improved social status. But the people who claim to hear the Hum having nothing to gain; some have even abandoned homes in a desperate effort to escape the noise [source: Deming].
What about tinnitus? As many as one in five people suffer from some degree of tinnitus, or ringing in the ears [source: American Tinnitus Association]. Maybe the Hum is literally in our heads. Again, not likely. Tinnitus suffers generally report a persistent ringing — both day and night — that registers in the highest audible frequencies, not the absolute lowest. There are types of tinnitus that sound like a hissing or static, but none that resemble a rumbling drone.
Another interesting theory is that Hum sufferers may simply have exceptionally sensitive hearing in the ultra-low-frequency range of 20 to 100Hz [source: Mullins and Kelly]. But if no acoustic instruments have been able to record the Hum, then what is it exactly that these sensitive ears are hearing?
Dr. David Baguley, head of audiology at Addenbrooke's Hospital in England, has researched the Hum for years, and blames the phenomenon on sensitive brains, not ears. Our sense of hearing is greatly heightened in times of stress, Dr. Baguley explained to BBC News in 2009. The brain cranks up the volume to detect the slightest threatening sound.
In the case of the Hum, Dr. Baguley believes people hear about the phenomenon in newspapers and around town and begin to fixate on a perceived noise. The incessant sound becomes a source of increasing frustration, disrupting sleep, and causing additional stress, which tricks the brain into turning up the volume even further. The solution, then, is to convince the brain to back down. With his own Hum patients, Dr. Baguley has found some success with simple relaxation techniques borrowed from psychology [source: Alexander].
What's that ringing in your ears? It's the sound of curiosity! For lots more fascinating facts about unexplained paranormal phenomena, check out the related HowStuffWorks articles on the next page.