The inner core of the Earth is made of a solid ball of metal
which is composed of Nickel and Iron.
Although it is the hottest layer of Earth, this still remains
in its solid state because of the pressure in the inner core. Metals like
Nickel and Iron remains solid at higher temperatures provided that the pressure
is still higher. Having these conditions, the effect of pressure override the
effect of temperature resulting to a solidified inner core.
