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.
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.