No human or machine has ever been 3,200 miles beneath Earth's surface because the depth, pressure and temperature make it inaccessible.
But scientists have long believed that our planet's inner core was solid, in contrast to the liquid metal region surrounding it.
Now that's been brought into question by a new study that claims the ball-shaped mass, which is responsible for Earth's magnetic field, contains both mushy and hard iron.
Scientists have long believed that our planet's inner core was solid. Now that's been brought into question by a new study that claims the ball-shaped mass contains both mushy and hard iron. Earthquake waves (pictured) were used as the basis for the research
Crust: To a depth of up to 70km, this is the outermost layer of the Earth, covering both ocean and land areas.
Mantle: Going down to 2,890km with the lower mantle, this is the planet's thickest layer and made of silicate rocks richer in iron and magnesium than the crust overhead.
Outer core: Running from a depth of 2,890- 5,150km, this region is made of liquid iron and nickel with trace lighter elements.
Inner core: Going down to a depth of 6,370km at the very centre of planet Earth, this region has been thought to be made of solid iron and nickel. But this new study suggests that it contains both mushy and hard iron.
The research has been led by Rhett Butler, a geophysicist at the University of Hawaii, who suggests that Earth's 'solid' inner core is, in fact, made up of a range of liquid, soft, and hard structures which vary across the top 150 miles of the mass.
Earth's interior is layered like an onion. The iron-nickel inner core is 745 miles in radius, or about three-quarters the size of the moon and is surrounded by a fluid outer core of molten iron and nickel about 1,500 miles thick.
The outer core is surrounded by a mantle of hot rock 1,800 miles thick and overlain by a thin, cool, rocky crust at the surface.
Because the inner core is so inaccessible, researchers had to rely on the only means available to probe the innermost Earth — earthquake waves.
'Illuminated by earthquakes in the crust and upper mantle, and observed by seismic observatories at Earth's surface, seismology offers the only direct way to investigate the inner core and its processes,' said Butler.
As seismic waves move through various layers of Earth, their speed changes and they may reflect or refract depending on the minerals,