What is at the center of the Earth, and how deep have we dug?

What the phenomenon is

Cut the Earth in half and four layers appear, from a thin rocky shell to a dense metal core. The crust is thinnest: about 30 kilometers (19 miles) thick under the continents and closer to 5 kilometers (3 miles) under the ocean floor, according to the U.S. Geological Survey. Below it sits the mantle, a hot layer of semi-solid rock roughly 2,900 kilometers (1,800 miles) thick.

The core itself has two parts. A liquid outer core of iron and nickel, about 2,200 kilometers (1,370 miles) thick, surrounds a solid inner core of the same metals, roughly 1,250 kilometers (780 miles) thick. Earth is about 12,750 kilometers (7,920 miles) across in total, so the solid center lies roughly 6,375 kilometers (3,960 miles) straight down from the surface.

The popular claim

The common picture of Earth's center is a molten, liquid interior all the way down, a glowing sea of rock someone could fall into. A related assumption is about access rather than composition: that with modern drilling technology, humans have gotten deep enough to approach it, or at least close to the mantle.

Neither holds up well against the evidence. The innermost layer is not liquid; it is solid metal, held rigid by pressure despite being the hottest part of the planet. And no drill has come remotely close to it — the deepest hole any country has ever completed barely breaks through Earth's thinnest layer, the crust.

How seismic waves mapped the inside of the Earth

Nobody has ever seen Earth's interior directly, and the deepest anyone has drilled covers only a small fraction of the way down. Almost everything known about the inside of the planet comes from seismic waves, the vibrations earthquakes send through the ground. The British Geological Survey explains that these waves bend and change speed as they cross materials of different density, much like light through a prism, letting scientists work out the interior without seeing it directly.

Two kinds of wave do the work. P-waves push and pull along their direction of travel and move through liquid or solid rock; S-waves shake side to side and cannot pass through liquid. Both travel through the mantle, showing it is solid. But S-waves disappear below a depth of about 2,900 kilometers (1,800 miles), and P-waves bend enough to leave a “shadow zone” between 103 and 143 degrees around the globe — evidence, the BGS notes, that the outer core must be liquid.

Inge Lehmann and the discovery of the inner core

The core itself turned out to have a second boundary inside it. In 1936, Danish seismologist Inge Lehmann was studying a 1929 earthquake near New Zealand when she noticed P-waves arriving inside the shadow zone, where the existing model said none should reach. She concluded there had to be another, denser layer inside the liquid core, and published her one-page paper, titled simply “P′,” in September 1936.

Acceptance took a few years — the British seismologist Harold Jeffreys remained unconvinced until 1939 — but by 1952, physicist Francis Birch had built the case that this inner layer was solid, crystalline iron. The boundary Lehmann found, between the liquid outer core and the solid inner core, is now called the Lehmann discontinuity in her honor. She died in 1993, at the age of 104.

How hot the center is

Temperature at the center has never been measured directly; it is calculated from how iron behaves under extreme pressure in the lab. In 2013, a team led by Agnès Dewaele of the French Alternative Energies and Atomic Energy Commission (CEA), working with the CNRS and the European Synchrotron Radiation Facility (ESRF), compressed tiny iron samples in a diamond anvil cell, heated them with lasers, and tracked their melting point using synchrotron X-ray diffraction.

Extrapolating those measurements to the pressure at the inner core's boundary, about 3.3 million atmospheres, the team put the melting point of iron there at roughly 6,000 degrees Celsius (about 10,800 degrees Fahrenheit), plus or minus 500 degrees. The result, published in Science on April 26, 2013, was about 1,000 degrees Celsius hotter than an estimate from similar experiments run twenty years earlier.

The deepest hole ever dug

The one place humans have tried to check any of this directly is the Kola Superdeep Borehole in northwest Russia. Soviet scientists began drilling in May 1970 and reached a final depth of 12,262 meters (40,230 feet, about 7.6 miles) in 1989, according to EarthDate, from the University of Texas at Austin's Jackson School of Geosciences. Drilling stopped for good in 1992, after the temperature at the bottom reached 180 degrees Celsius (356 degrees Fahrenheit), softening the rock until it behaved like plastic.

It remains the deepest hole ever made by any country. And it came nowhere near anything resembling the mantle or core: EarthDate calculates that reaching Earth's center from the bottom of the Kola hole would take drilling about 520 times deeper still, meaning the world's deepest borehole covered only about 0.19 percent of the distance down.

What is still debated

The layered structure itself is settled science, confirmed by nearly a century of seismic recordings from earthquakes all over the world. What is still being worked out is how the inner core behaves over time. In 2024, a team led by John Vidale at the University of Southern California compared seismic waves from 121 earthquakes that repeated near Antarctica's South Sandwich Islands between 1991 and 2023.

They found that the inner core had begun rotating slightly slower than the rest of the planet around 2010 — the first time that had happened in roughly 40 years, likely tied to the churn of the liquid outer core and the gravitational pull of the mantle above. The study was published in the journal Nature on June 12, 2024.

A follow-up study from the same USC-led team, published in Nature Geoscience on February 10, 2025, went further: seismic data suggested that the near-surface of the inner core is not fixed in shape but can deform, as the liquid outer core disturbs it. “What we're observing in this study for the first time is likely the outer core disturbing the inner core,” Vidale said. How these shifts affect the length of Earth's day is still being worked out.

Verdict

At the center of the Earth is a solid ball of iron and nickel roughly 1,250 kilometers (780 miles) thick, under pressure and heat never measured directly. Real or Legend has filed the layered structure of the Earth as confirmed, resting on a century of consistent seismic evidence since Lehmann's 1936 discovery of the inner core. Left open is how that inner core moves and changes shape, which geophysicists are still measuring, and which no borehole ever drilled has come close to reaching.