Earth's surface feels solid and familiar, yet thousands of kilometers beneath it lie slowly deforming rock, a liquid metal outer core, and a solid inner core under enormous pressure. Scientists have mapped these layers without ever reaching them directly.

The Kola Superdeep Borehole reached a depth of about 12.3 kilometers below the surface, while Earth's average radius is roughly 6,371 kilometers. That tiny fraction of the distance shows why researchers depend on indirect evidence, particularly earthquake waves, magnetic measurements, and laboratory experiments, to investigate the deep interior.

Earthquakes Work Like Giant Flashlights

When an earthquake occurs, it sends seismic waves through Earth. Two important types are P-waves, which compress and expand material in the direction they travel, and S-waves, which move material sideways.

P-waves can pass through solids and liquids, although their speed varies with the material. S-waves require a material that can resist shearing, so they cannot travel through a liquid.

By recording seismic waves at stations around the world, scientists can identify where waves slow down, bend, reflect, or disappear. These patterns reveal boundaries between Earth's internal layers.

In 1906, British seismologist Richard Oldham identified evidence for Earth's core by studying unusual seismic travel times. Later research established that the outer core is liquid, particularly because it does not transmit ordinary S-waves and because P-waves slow sharply upon entering it.

Today, scientists understand the outer core as a layer of liquid metal, primarily iron with nickel and lighter elements, roughly 2,260 kilometers thick. It surrounds the solid inner core.

Seismic measurements provide far more than a simple map of the layers. Differences in wave speeds help researchers investigate temperature, composition, and structure deep inside the planet.

The Inner Core Is a Solid Metal Mystery

Earth's inner core is extraordinarily hot, with temperatures commonly estimated at around 5,000 to 6,000 degrees Celsius. Yet it remains solid because the immense pressure raises the melting temperature of its iron-rich material.

In 1936, Danish seismologist Inge Lehmann identified evidence for a distinct inner core after examining seismic waves arriving in regions where simpler models predicted very weak signals. Her work revealed an additional boundary deep inside Earth.

Further seismic research later established that the inner core behaves as a solid.

The inner core is not completely uniform. Seismic waves generally travel faster along certain directions than others, a property called seismic anisotropy. Scientists investigate whether this reflects the alignment of iron crystals, structural variations, or other effects.

Some models also suggest that the innermost portion of the inner core has properties distinct from the surrounding solid metal.

Another question concerns its rotation. By comparing seismic waves from earthquakes that occurred in nearly the same locations at different times, researchers have found evidence that the inner core can change its rotational position relative to the mantle.

A 2024 study by John Vidale and colleagues in Nature found evidence of a change in relative rotation using repeating earthquakes near the South Sandwich Islands. The details of this motion and the processes driving it remain subjects of research.

Magnetic Clues From Ancient Rocks

Earth's magnetic field provides another way to investigate its deep interior. The field is generated mainly by the movement of electrically conducting liquid metal in the outer core, through a process called the geodynamo.

As certain volcanic rocks cool, magnetic minerals can record the direction and strength of Earth's magnetic field. Some sedimentary rocks can also preserve magnetic information as they form.

By studying and dating these rocks, scientists have reconstructed parts of Earth's magnetic history over hundreds of millions of years, and even billions.

One important discovery is that Earth's magnetic field has reversed polarity many times. During a reversal, magnetic north and south exchange their roles. The most recent complete reversal occurred roughly 780,000 years ago.

Magnetic records also reveal changes in field strength and structure. These variations offer clues about the behavior of the liquid outer core, although they cannot provide a simple, direct measurement of its temperature or convection speed.

Together with seismic data and computer simulations, paleomagnetic evidence helps researchers understand how Earth's magnetic field has evolved.

Mantle Plumes and the Heat Budget

Between Earth's crust and core lies the mantle, a layer of mostly solid silicate rock about 2,900 kilometers thick. Although the mantle is solid on short timescales, its rocks can deform and flow extremely slowly over millions of years.

Heat from Earth's interior helps drive mantle convection, which is part of the system responsible for plate tectonics. Plate motion also involves forces such as the sinking of cold, dense slabs of oceanic lithosphere.

Some volcanic regions far from plate boundaries may be associated with mantle plumes, columns of unusually hot material rising from deep within Earth. Hawaii is a widely studied example, while the origins of volcanism beneath places such as Yellowstone remain subjects of investigation.

Scientists investigate these structures using seismic tomography, a method that combines measurements from many earthquakes to construct three-dimensional models of Earth's interior.

Seismic waves often travel more slowly through hotter regions of mantle rock, although temperature is not the only factor affecting their speed. Composition, partial melting, and mineral properties also matter.

Tomographic studies have identified broad regions of relatively slow seismic velocities beneath some volcanic hotspots, consistent with deep mantle upwellings. However, the exact shapes, depths, and origins of individual plumes are still debated.

What We Still Don't Know

Scientists have developed a detailed model of Earth's internal layers, but major questions remain about their exact composition, temperature, and movement.

The inner core is thought to consist mainly of iron with some nickel and lighter elements, yet the identity and proportions of those lighter elements remain uncertain. Temperatures near the core-mantle boundary are also difficult to determine precisely.

Researchers have obtained mantle rocks through volcanic eruptions and from places where mantle material has been exposed at Earth's surface. However, these samples mostly represent relatively shallow parts of the mantle. No one has directly sampled the deep mantle or Earth's core.

One particularly intriguing mystery involves two enormous regions of unusually low seismic shear-wave speeds near the base of the mantle, beneath Africa and the Pacific.

Known as large low-shear-wave velocity provinces, these structures may differ from the surrounding mantle in temperature, composition, or both. Some hypotheses suggest they contain ancient material preserved from early in Earth's history, but their origins remain uncertain.

Better seismic measurements, high-pressure laboratory experiments, and improved computer models are helping researchers test these competing explanations.

Scientists cannot see Earth's core directly, but they can measure how seismic waves cross it, how its magnetic field changes, and how materials behave under extreme pressure.

These methods have revealed a liquid outer core, a solid inner core, and a slowly moving rocky mantle. The remaining mysteries concern how those layers interact and what their hidden structures can tell us about Earth's history.