Earthquakes, explosions and observations of Earth's ever-changing magnetic field are helping scientists open up a new window on the heart of our planet.
When Jules Verne wrote A Journey to the Centre of the Earth over 100 years ago, he imagined a place of glowing crystals and a turbulent sea, complete with prehistoric animals and giant mushrooms.
What was actually beneath our feet was a complete enigma. Even to this day scientists astonishingly know more about the rings of Saturn than they do about the core of our own planet.
But that is beginning to change. "We're at a golden age in terms of the real discovery of the bulk of the deep Earth," says seismologist Professor Rick Aster.
And remarkably, not everything Verne imagined was wrong.
Continue reading the main story
FIND OUT MORE
- Horizon: The Core is on BBC Two at 9pm on Wednesday 31st August
- Catch up via iPlayer or watch clips at the Horizon website
Wanting to discover the truth about the centre of our world is as basic a human urge as wondering what is on the Moon, although the latter has proved far easier to explore.
But scientists are also fascinated by the Earth's core because it is responsible for creating our planet's magnetic field which is vital to life.
As a tool for navigation, it helps honey bees find their hive, while sea turtles, birds and butterflies use it to migrate over long distances.
The magnetic field also acts as a protective barrier between us and some of the dangers of space, shielding us from radiation in the solar wind.
Physically travelling to the core has proved a non-starter though, because of the rapidly increasing pressures and temperatures.
Even with remote drilling, the deepest we have managed to penetrate is 12km at the Kola Superdeep Borehole in Russia - a measly 0.2% of the way to the centre of the Earth.
We may have very big crystals at the centre of the Earth, maybe up to 10km”
Professor Kei Hirose
Tokyo Institute of Technology
But seismology has allowed scientists to sense right into the core of the planet. The seismic waves generated during major earthquakes travel from one side of the Earth to the other, allowing scientists to build up a picture of the interior.
Seismology is "the killer application", says Aster, showing us the Earth has a molten outer-core, "an enormous ocean of white hot molten metal that's almost as runny as water".
This core is as large as Mars. But - like a planetary Russian doll - another core was found within this one. An inner-core - a solid metal ball almost the size of the Moon.
Scientists believe the solid inner core is made of an iron-nickel alloy.
To understand what form it might take under the extreme conditions at the centre of the Earth, Professor Kei Hirose set himself a seemingly impossible challenge: recreate the conditions of the core in his lab at the SPring-8 synchrotron near Osaka, Japan. After 10 years of trying, he has finally succeeded.
He has created an incredibly powerful vice using the tips of two diamonds. Between them he has pressurised a sample of iron-nickel to three million times atmospheric pressure and heated the sample to about 4,500C.
Under these extraordinary conditions, the crystal structure of iron-nickel alloy changed and the crystals rapidly grew in size. "We may have very big crystals at the centre of the Earth, maybe up to 10km," says Hirose.
These crystals would all align "like a forest", says Hirose, pointing at the poles.
The bulk of the Earth's magnetic field is generated not in the inner-core but in the molten metal of the outer-core. This acts as a massive electromagnetic dynamo powered by the Earth's rotation and the long-term cooling of the planet.
But although the basic principle is understood, the details of how the molten metal moves are a mystery. As Earth rotates and loses heat from the centre, complex patterns of flow are created within this vast ocean.
"You might think of the core like the atmosphere of the Earth, being a very restless place with storms and fronts and bad weather," says geophysicist Professor Dan Lathrop from the University of Maryland. He has built himself a massive model of the core to help explain something strange about the field - it is never fixed but constantly fluctuating.
The Earth's magnetic field has been steadily weakening over the past 180 years. And there is one patch that is weakening faster than any other. It is an area scientists have dubbed the "South Atlantic Anomaly", which sits over the South Atlantic and the centre of South America.
It is a known hazard for spacecraft because it creates a dip in the field, allowing charged particles into the orbit of satellites and upsetting their electronics and instrumentation.
But what some scientists suspect is that it could be much more than an inconvenience to satellite operators - it could be the first indication of a profound change in Earth's magnetic field.
When scientists mapped the Earth's magnetic field down to the level of the outer-core, they discovered that under the South Atlantic Anomaly the simple north-south divide we know at the surface had broken down. There are patches where the field has actually flipped and points north instead of south.
Using his weather analogy, Lathrop believes "a particularly violent or unusual patch of weather" in the molten metal of the outer-core is responsible for reversing the field.
If these patches continue to deepen and spread, the entire Earth's magnetic field could reach a tipping point and flip, he believes.
It is not something that would happen overnight - it could take thousands of years, during which period the field would be pretty confused.
The magnetic poles could wander to the equator for example, and take with them the spectacular Northern Lights. It would not be out of character - the shifting flows of the core have reversed Earth's field hundreds of times before.
"It's not a question of if the Earth is going to reverse the magnetic field, but when," says Lathrop.
Exactly when this might be remains one of the core's many mysteries. But after centuries of speculation, scientists are finally beginning to understand this great wonder lying 6,000km beneath our feet.