No human or machine has ever been 3,200 miles below the Earth’s surface because depth, pressure and temperature make it inaccessible.

But scientists have long believed that the inner core of our planet was solid, in contrast to the liquid metal region that surrounds it.

Now, a new study has been questioned claiming that the spherical mass responsible for the Earth’s magnetic field contains both porridge and hard iron.

Scientists have long believed that the inner core of our planet was solid. Now it has been questioned by a new study claiming that the spherical mass contains both mushy and hard iron. Earthquake waves (pictured) were used as the basis for the research

The research has been led by Rhett Butler, a geophysicist at the University of Hawaii, who suggests that the Earth’s ‘solid’ inner core actually consists of a series of fluid, soft and hard structures that vary across the top 150 miles of mass.

The earth’s interior is layered like an onion. The iron-nickel inner core is 745 miles in radius, or about three-quarters of the size of the moon, and is surrounded by a liquid outer core of molten iron and nickel that is about 1,500 miles thick.

The outer core is surrounded by a mantle of hot rock 1,800 miles thick and superimposed by a thin, cool, rocky crust at the surface.

Because the inner core is so inaccessible, scientists had to rely on the only means available to study the innermost earth – earthquake waves.

‘Illuminated by earthquakes in the crust and upper mantle, and observed by seismic observatories on the Earth’s surface, seismology provides the only direct way to study the inner core and its processes,’ Butler said.

As seismic waves move through different layers of the Earth, their velocity changes and they can reflect or break depending on the minerals, temperature and density of this layer.

To better understand the functions of the Earth’s inner core, Butler and his co-author Seiji Tsuboi, a researcher at the Japan Agency for Marine-Earth Science and Technology, used data from seismometers directly opposite the site of an earthquake.

They used Japan’s Earth Simulator supercomputer to assess five matings to largely cover the inner core area: Tonga and Algeria, Indonesia and Brazil, and three between Chile and China.

An intersection of the Earth’s interior shows the inner core (red) and the liquid iron outer core (orange). Seismic waves move faster through the Earth’s inner core between the North and South Poles (blue arrows) than over the equator (green arrow)

Because Earth’s inner core is so inaccessible, scientists had to rely on the only means available to study the innermost earth – earthquake waves (stock image)

‘In stark contrast to the homogeneous, soft iron alloys considered in all soil models in the inner core since the 1970s, our models suggest that there are adjacent areas of hard, soft and liquid or mushy iron alloys in the upper 150 miles of the inner core, ‘said Butler.

‘This puts new constraints on the Earth’s composition, thermal history and evolution.’

The scientists said that this discovery of the diverse structure of the inner core could offer important new information about the dynamics at the boundary between the inner and outer core that affect the Earth’s magnetic field.

‘Knowledge of this boundary state from seismology can enable better, predictable models of the geomagnetic field that protect and safeguard life on our planet,’ Butler said.

Scientists now plan to model the inner core structure in more detail using the Earth Simulator supercomputer so they can see how it compares to different properties of the Earth’s geomagnetic field.

The research has been published in the journal Science Direct.