The planet's internal structure comprises four layers: a rocky crust on the outside, then a rocky mantle, an outer core made of magma and a solid inner core.
This metallic inner core, about 1,500 miles (2,440) wide, was discovered in the 1930s, also based on seismic waves traveling through Earth.
Scientists in 2002 proposed that lurking within this inner core was an innermost section separate from the rest, akin to a Russian Matryoshka nesting doll.
The researchers were able to differentiate the two regions because the seismic waves acted differently between them.
"The latent heat released from solidifying the Earth's inner core drives the convection in the liquid outer core, generating Earth's geomagnetic field," Pham said.