Mystery 'hum' that has baffled people around the world for decades is recorded in the ocean for the first time - but scientists still can't explain what it is

Experts have known that our planet gives off a low frequency hum for decades
It is caused by tiny vibrations in the Earth but its source is a mystery
Previous recordings of the signal have come from land-based equipment
Now scientists have captured the hum deep in the ocean for the first time
The reading could help experts to finally determine what causes the noise

By Harry Pettit For Mailonline

Published: 09:31 EST, 8 December 2017 | Updated: 10:46 EST, 8 December 2017

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A strange hum that has baffled people all over the world for decades has been recorded in the ocean for the first time.

The low frequency hum has been known about for nearly half a century- but its source remains a mystery.

Scientists claim the sound is inaudible to humans, but thousands of people still insist they have heard it.

A website, called ‘The Hum’, has reports about the noise dating back as far as the 1960s.

Theories about what causes it range from mating fish, a secret project to tunnel under the Earth and the presence of aliens.

Now, by measuring the sound using underwater seismometers positioned across the globe, scientists may be able to finally figure out what it.

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An eerie humming noise emitted by Earth has been captured deep in the ocean for the first time, but scientists still can't explain what causes it. Experts have known that our planet gives off a low frequency hum for decades, though it is inaudible to humans (stock image)

WHAT CAUSES THE HUM?

Experts have known that our planet gives off a low frequency hum for decades, though it is inaudible to humans.

Researchers have previously suggested the hum is caused by the constant pounding of waves on the ocean floor, or by currents whisking over continental shelves.

Other studies have suggested the noise is the result of atmospheric turbulence, as it is stronger in the northern hemisphere's Pacific Ocean during the northern winter, and southern oceans during the southern hemisphere's winter.

The researchers found that the amplitude of the hum did not match up with seasonal changes, meaning atmospheric turbulence can only partially explain the sound.

It's not clear what's causing the hum, but scientists say it may be the result of tiny vibrations in the Earth called 'free oscillations' that can only be picked up by sensitive equipment.

The vibrations are triggered by constant minuscule expansions and contractions of our planet constantly, though scientists still don't understand their source.

Earthquakes have been known to cause the Earth to emit similar vibrations for extended periods, but the hum only occurs in the absence of seismic activity.

The first attempt to detect this hum was made in 1959, but it wasn't until 1998 that scientists had conclusive proof it exists.

Many observations of the hum have been taken since then from land-based seismometers.

And many people have reported hearing it on The Hum website, although scientists claim this is impossible.

MacPherson said: ‘Most people find my website because they are searching for the source of an unusual low-frequency sound.

'The classic description is that it sounds like there is a truck idling outside your home. For some people, it is a deep and distant droning bass tone. Some people perceive the sound as a rumbling noise.

'The sound is louder indoors than outdoors, and louder late at night than during the afternoon. It can suddenly appear or disappear for days or months.

‘People of all ages can be affected, although the incidence among children is very low. In the more serious cases, the Hum can affect people’s quality of life. In a number of documented cases, the torment of the noise has been life-altering.’

Experts at France's Paris Institute of Earth Physics have recorded a humming noise from the bottom of the ocean.

Previously this has been impossible because seismometers measure movement, meaning tidal interference obscures any readings of the tiny signal.

Experts have previously suggested the hum is caused by the constant pounding of waves on the ocean floor, or by currents whisking over continental shelves. Other studies have suggested the noise is the result of atmospheric turbulence (stock image)

The team studied 11 months' worth of observations from 57 seismometer stations on the seafloor in the Indian Ocean east of Madagascar.

They picked the two stations with the highest quality readings and painstakingly removed any interference from the data.

By cross-referencing what was left with measurements from terrestrial stations, the team realised the signal that remained was the hum.

The researchers found the noise is almost 10,000 times below the 20 hertz threshold of human hearing.

THE STUDY

The team studied 11 months worth of observations from 57 seismometer stations on the seafloor in the Indian Ocean east of Madagascar.

They picked the two stations with the highest quality readings and painstakingly removed any interference from the data.

By cross-referencing what was left with measurements from terrestrial stations, the team realised the signal that remained was the hum.

The researchers found the noise is almost 10,000 times below the 20 hertz threshold of human hearing.

The team studied 11 months' worth of observations from 57 seismometer stations on the seafloor (pictured)

Experts have previously suggested the hum is caused by the constant pounding of waves on the ocean floor, or by currents whisking over continental shelves.

The researchers found that the amplitude of the hum did not match up with seasonal changes, meaning atmospheric turbulence can only partially explain the sound.

'The Earth's hum is the permanent free oscillations of the Earth recorded in the absence of earthquakes,' the team, led by Dr Martha Deen, wrote.

'The hum was proposed to be induced by acoustic resonance between the atmosphere and the solid Earth, but this can only explain part of its [loudness].'

The researchers say studying the hum via ocean floor seismometers could help to determine its origins, particularly if currents or waves are responsible.

They also say their technique could help map Earth's interior, which is usually done using sporadic seismic data from earthquakes.