Here's why:

* Composition: The inner core is primarily made up of iron (Fe) with a small percentage of nickel (Ni).

* Pressure: The immense pressure from the weight of all the layers above it squeezes the iron atoms so tightly together that it becomes solid, even though the temperature is extremely high (around 5,200° Celsius).

* Density: The combined effect of the heavy elements and intense pressure results in a density of around 13 g/cm³, making it the densest layer of the Earth.

For comparison, the Earth's average density is about 5.5 g/cm³.