The Earth’s inner core is a sphere of mostly iron around 2,442 km (1,517 miles) across. At 5,000-6,000 C (9,000 – 11,000 F) it is solid rather than liquid, due to the immense pressures in the Earth’s interior. Many geologists now believe that this gigantic ball of iron is actually a single crystal. This is due to differences in the bahaviour of seismic waves passing through it in different directions. This huge hot ball of iron is about three-quarters the size of the Moon, and has a mass of around one hundred million million million tonnes.