Back in the 1970s, about 800 people in Bristol, England started saying that, both during daytime and during nighttime, they could hear a mysterious hum that would not go away and that was driving them crazy.
Over the years, the hum has been reported in Taos, New Mexico, Windsor, Ontario, Sydney, Australia and in Largs, Scotland. More often than not, it is heard in rural and suburban regions.
This is probably because such areas are quieter than crowded cities, and it is easier for the people inhabiting them to detect it.
The lack of noise pollution could also explain why the hum appears to get louder at night.
Those who hear this hum say that it is of low frequency, and that is resembles a throbbing or rumbling sound, Live Science reports.
Some argue that hearing the hum is almost like listening to a diesel engine that's idling.
Researchers say that, because this hum is constantly playing in their heads, some people have come to suffer from nausea, headaches, dizziness and even nosebleeds.
Needless to say, they're also unable to get enough sleep, seeing how the hum makes them spend most of the night tossing and turning in their beds.
Oddly enough, this weird sound is only detectable indoors. What's more, just 2% of the people living in regions where the hum has been reported can pick it up.
These “lucky few” are all between the ages of 55 and 70. Scientists have nicknamed them “hearers” or “hummers,” the same source informs us.
Presently, researchers are unable to say where the hum comes from and what causes it. They've spend many years investigating this phenomenon, yet their efforts have proved futile.
Since the people who hear it are perfectly healthy, it is unlikely that the hum can be blamed on some peculiar medical condition that only affects a handful of them.
Some scientists say that the hum might be caused by electrical power lines, wireless devices or some other pieces of equipment. Others point the finger at seismic activity.
Whatever its cause, it appears that the hum is here to stay. What's more, it will probably be a while before researchers determine its cause.