While the mantle is the largest and most voluminous layer, the immense pressure at the Earth's center compresses the inner core, primarily made of iron and nickel, into a solid state despite incredibly high temperatures. This pressure makes it the hardest layer, with an estimated hardness surpassing that of any known material on the surface.

Here's why:

* Extreme pressure: The inner core experiences immense pressure from the weight of all the layers above it. This pressure is estimated to be over 3.3 million times the pressure at sea level.

* Solid state: Despite its high temperature (estimated to be around 5200°C), the intense pressure forces the iron and nickel atoms to pack tightly together, creating a solid state.

* Strong metallic bonds: The metallic bonds between iron and nickel atoms are incredibly strong, contributing to the inner core's rigidity.

While we can't directly sample the inner core, our understanding of its properties comes from studying seismic waves that travel through the Earth. These waves reveal that the inner core is indeed a solid and the hardest layer of our planet.