Fifth layer inside the Earth's core has just been found!
A team at Australian National University (ANU) has found evidence of a new fifth layer to the planet, an iron-nickel alloy ball in the inner core.

The scientists found the hidden core by studying seismic waves that travel up to five times across the Earth's diameter – previous studies only looked at single bounces. The earthquake waves probed places near the center at angles that suggested a different crystalline structure deep inside.
Source: engadget