With more than 5,000 km (3,000 mi) of rock between us and the Earth's core, it's hard to get a clear look at what's going on down there. It's long been thought that the inner core is a solid ball of iron, but direct evidence is hard to come by. Now, researchers from the Australian National University (ANU) have detected a type of seismic wave passing through the core that only propagates through solid objects.

The idea that the Earth has a heart of solid iron has been around since the late 1930s, but proof has remained elusive. A recent theory even explained how it can remain solid at temperatures on par with the surface of the Sun, using a supercomputer to show that the heat and pressure allows the iron atoms to stay in a stable crystal structure.

Still, scientists have their ways to peer inside. After large earthquakes strike near the surface, they give off seismic waves that ripple through the surrounding material, and measuring how they pass through or bounce off different materials can paint a picture of what hides beneath. In a way, it's kind of like doing an ultrasound image of an unborn baby. Using this technique, a recent study found that the inner core might even have its own distinct inner core.

The researchers on the new study used a similar technique that they call the correlation wavefield method. Rather than the direct wave arrivals, this technique looks at the similarities between signals picked up by two receivers in different locations, in the hours following a major earthquake.

"We're throwing away the first three hours of the seismogram and what we're looking at is between three and 10 hours after a large earthquake happens," says Hrvoje Tkalčic, co-author of the study. "We want to get rid of the big signals. Using a global network of stations, we take every single receiver pair and every single large earthquake – that's many combinations – and we measure the similarity between the seismograms. That's called cross correlation, or the measure of similarity. From those similarities we construct a global correlogram – a sort of fingerprint of the Earth."

From this technique, a strange picture emerges. The team was able to detect shear waves (or J waves) on their journey through the center of the Earth, which are extremely small and hard to spot. Since these waves only travel through solid objects, the researchers say that's solid evidence (excuse the pun) that the core is solid. That said, it doesn't seem to be completely rigid – it's actually a bit squishy.

"We found the inner core is indeed solid, but we also found that it's softer than previously thought," says Tkalčić. "It turns out – if our results are correct – the inner core shares some similar elastic properties with gold and platinum. The inner core is like a time capsule, if we understand it we'll understand how the planet was formed, and how it evolves."

The research was published in the journal Science.