Scientists have discovered how to use the natural hum inherent to the Earth to see deep within the planet's interior.
Various processes, including ocean swells and atmospheric disturbances set up very low frequency vibrations - less than 0.01Hz - that propagate through the planet. Because the character of these waves is highly random it's possible to identify and follow specific signature sounds as they travel around the planet. The sounds change as they pass through different geological structures, so by tuning into them from many different places on the planet's surface you can build up a picture of what must lie beneath.
The work has been published in the current edition of the journal Science by Kiwamu Nishida at the University of Tokyo and the studies are based on recordings made between 1986 to 2003 by 54 seismic stations dotted around the world.
This approach is very similar to another method that scientists use to see inside the Earth, which is by looking at how vibrations triggered by Earthquakes spread around the planet. The data generated by the two techniques agrees very well, proving that it works, but the new approach offers the additional advantages of continuous sampling, the ability to see very deep inside the Earth - down as far as 500km say the scientists - and could even be used to reveal the internal structures of other planets.
According to the team atmospheric disturbances would set up a similar hum on Mars, allowing scientists to map the planet's interior.