Last week, Japanese authorities were baffled by a rusted metal ball that washed up on Enshu beach. After X-ray scans, they determined that it was hollow and not threatening to the public, but nevertheless the 1.5-metre sphere prompted numerous theories, ranging from UFOs to espionage. As it turns out, though, this particular metal ball was the least of the conspiracy theorists’ worries.

The same week, on February 21, scientists claimed to have discovered a solid, 400-mile-wide iron ball at the centre of Earth, distinct from the one that we’ve known about since Inge Lehmann’s landmark discovery in 1936. In fact, the study, published in Nature Communications, rewrites what we’ve always been told about the structure of our home planet – namely, that it’s made up of four distinct parts, the crust, the mantle, and the inner and outer cores.

The idea that Earth is made up of five layers was first introduced by two seismologists, Miaki Ishii and Adam Dziewonski, in the early 2000s, based on some unusual patterns in seismic waves generated by earthquakes that passed through the inner core. At the time, though, they didn’t have enough data to back up their argument.

Now, with the help of new seismometers and techniques – such as measuring waves that bounce back and forth through the Earth’s core, up to four times – two Australia-based scientists, Hrvoje Tkalcić and Thanh-Son Pham, appear to have proved the hypothesis.

Apparently, the discovery that Earth is made up of five layers could help solve some age-old mysteries about the planet and how it was formed. Is the innermost core actually a buried spaceship, harbouring alien beings? Is it a giant iron egg, ready to hatch? OK, probably not. However, it is quite interesting in scientific terms, acting as a “time capsule of our planet’s history”.

In particular, the inner core is interesting because it’s relatively young, in geological terms (i.e. about 600 million to one billion years old, in comparison with the 4.5-billion-year-old Earth). Studying its complex composition could help to unearth some historic changes, such as an event that split the inner core into two parts, as well as supporting new theories about the planet’s magnetic field. 

Then, there’s the question of the core slowing down or changing direction, as estimated by scientists earlier this year. Could the big metal ball at the centre of the Earth hold the answers? Our knowledge about the planet beneath our feet is notoriously hazy and, while the core lies 4,000 miles deep, we’ve barely scratched the surface, so it looks like we’ll have to wait on seismologists to find out more.