Have you heard 'the hum'? Mystery of Earth's low droning noise could now be solved

Scientists have confirmed the cause of a strange humming noise that emanates from the Earth and has baffled people for more than forty years - and was even a factor in one reported suicide.

The noise has been talked about worldwide and also made local newspaper headlines in the UK. It is often referred to as a "phenomenon" and "the hum", usually prefixed with the location of where it is heard.

In Britain, the most famous example was the "Bristol hum" that made the news in the late 1970s. One newspaper asked readers in the city: "Have you heard the Hum?" and at least 800 people said they had - according to the BBC - and some had suffered headaches and nosebleeds from it.

It has been described like "a diesel car idling in the distance" by a BBC interviewee and the maddening sound has driven people stir-crazy in trying to figure it out. Especially when they can only hear it at home and during the night.

People living on the south coast have complained this week of a constant and low-pitched sound for which they have found no cause - as reported by Plymouth Herald.

It has been mistaken for leaking pipes, phone masts, wind farms, low-frequency submarine communications and even mating fish.

"For the first few years I lost sleep, couldn't concentrate and was unable to do anything. I was constantly in tears, which put a great strain on my husband. It has changed me from an active, creative person to a stifled, angry pessimist," a woman told The Independent back in 1994.

Doctors blamed patients' abilities to hear it on tinnitus, until Addenbrooke's Hospital in Cambridge had confirmed sometime in the 1990s that the cause is external.

However, the search for the truth could now be over as researchers claim that microseismic activity from long ocean waves impacting the sea bed is what makes our planet vibrate and produces the droning sound.

The pressure of the waves on the seafloor generates seismic waves that cause the Earth to oscillate, said Fabrice Ardhuin, a senior research scientist at Centre National de la Recherche Scientifique in France.

The continuous waves produce sounds lasting from 13 to 300 seconds. They can be heard by a relatively small proportion of people - who are sensitive to the hums - and also by seismic instruments.

"We have made a big step in explaining this mysterious signal and where it is coming from and what is the mechanism," Ardhuin said of the study published in Geophysical Research Letters, a journal of the American Geophysical Union.

Understanding the ringing could also help researchers gain a better knowledge of the Earth's structure, he added.

Microseismic waves penetrate through the Earth's mantle so recording these waves could give scientists a much more detailed picture of what lies beneath.

Discovering fainter seismic signals could also allow scientists to better detect small or faraway earthquakes.