The Earth constantly emits an eerie "hum", which is not clearly audible to us. Previous attempts to record the mysterious signal all involved land-based observations, but now scientists have finally captured the mysterious signal deep under the ocean.

Although earthquakes have been known to cause the Earth to emit vibrations for extended periods of time, this mysterious hum occurs even in the absence of any significant seismic activity. The low-frequency signal is essentially the Earth's permanent free oscillations, which can only be recorded using sensitive instruments.

The source of the constant low-frequency vibrational signal is yet to be determined. However, by measuring the signal at the ocean floor, scientists may be able to finally figure out what causes the hum.

ScienceAlert reports that the first attempt to detect the Earth's hum was made in 1959. However, it was only in 1998 that scientists were able to confirm its existence. Since the 1998 study, there have been numerous observations that confirm the authenticity of the signal. Unlike the recent study, which has been published in the journal Geophysical Research Letters, previous observations were made using seismometers on land.

The new research is important because one of the long-standing theories explaining the phenomenon involves the constant pounding of waves on the ocean floor. Previously, scientists have also suggested that powerful ocean waves can shake the Earth as they pitch over underwater continental shelves. Researchers have also previously speculated whether ocean waves travelling along shorelines or under mountains could generate seismic activities that could be connected to the source of the mysterious hum.

Since this is the first time that the Earth's hum has been recorded deep under water, this theory could be further explored by researchers using the data gathered by the underwater observations of the signal.

Since 70% of the Earth's surface is covered by water, measuring the hum under water may provide scientists the ability to analyse the puzzling phenomenon with vast amounts of global data.

To capture the hum, a team of researchers led by Martha Deen, a geophysicist at the Paris Institute of Earth Physics in Paris, spent 11 months gathering data from 57 seismometer stations deep under water in the Indian Ocean, east of Madagascar. After eliminating all the ambient ocean signals, the scientists compared the remaining data with observations of the hum made by terrestrial stations, to discover that they had a match.

The researchers found that the Earth's hum is nearly 10,000 times below the threshold of human hearing – which begins at 20 hertz. The scientists also believe that the findings of the study may help map the Earth's interior.