Here's why:
* Composition: The inner core is primarily composed of iron with a small percentage of nickel.
* Pressure: The immense pressure at the Earth's center forces the iron atoms to pack incredibly tightly together. This extreme pressure is what makes the inner core solid despite its extremely high temperature.
* Density: The combination of heavy elements and immense pressure results in a density of around 12.6-13.1 g/cm³. This is significantly denser than any other layer of the Earth.
For comparison, the density of the outer core is around 10-12 g/cm³, the mantle is around 3.3-5.7 g/cm³, and the crust is around 2.7-3.0 g/cm³.
