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Scientists have captured a mysterious hum coming from deep inside the Earth, but they are still no closer to working out what it is.

It has long been known that the Earth constantly generates a low-frequency vibrational signal.

The first attempt to detect this hum was made in 1959, but it wasn't until 1998 that scientists finally proved its existence.

Since then, there have been hundreds of attempts to record the Earth's hum, but they have all been made using seismometers on land.

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(Image: Getty)

Now researchers have captured the hum for the first time using seismic instruments at the bottom of the sea.

The researchers first gathered data from 57 seismometer stations located in the Indian Ocean, east of Madagascar, between 2012 and 2013.

They then applied a combination of techniques to remove interference from ocean infragravity waves, currents and electronic glitches, and corrected for the signal generated by any earthquakes.

Eventually, they were able to determine that the Earth's natural vibration peaks at several frequencies between 2.9 and 4.5 millihertz.

These vibrations can't be heard by people, because they are approximately 10,000 times smaller than the lower hearing threshold of the human ear (20 hertz).

However, the fact that they have been detected at the bottom of the ocean, and around 70% of the planet's surface is covered by water, suggests that the hum is present across the entire globe.

The research team, led by Martha Deen at the Paris Institute of Earth Physics, hopes the new data will provide clues as to the source of the hum.

Seismologists have proposed many different theories to explain the continuous vibration - from atmospheric disturbances to ocean waves moving over the sea floor.

(Image: Reuters)

However, the study, published in Geophysical Research Letters, concludes that these theories can only explain part of the vibration.

Deen and her co-authors believe the data could also help scientists map the interior of Earth with more detail and accuracy than ever before.

Scientists traditionally study the Earth's interior during earthquakes, but this mean they are severely limited to specific times and areas where quakes commonly occur.

Using the hum signal as a source of seismic waves would avoid this problem, because it is generated continuously, and can be detected across the planet's surface.

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