The layer of the Earth that is solid is the inner core. It is mostly made up of a solid ball of iron-nickel alloy and other elements with a radius of about 760 miles or 1, 220 kilometers. Though it is a solid layer, its temperature is as hot as the sun’s surface that is approximately 5, 430 °C/9,806 °F or 5,700 K.
The inner core was discovered by Inge Lehmann, a Danish seismologist, when she was studying earthquakes’ seismographs in New Zealand. It was resolved that the outer core is a molten layer when compressional waves can pass through while shear waves cannot pass through the inner layer.