We know the Earth has liquid center by mapping S waves, also known as secondary waves or shear waves. Earth’s center has two portions: the outer core and the inner core – it was thought that the outer core is liquid iron and the outer core is solid iron. To check if this hypothesis is correct, scientists mapped S waves because they found out that S waves cannot travel through liquid.
It was through this process that the early hypothesis about the inner core being liquid iron and the outer core being solid iron is proven wrong. Scientists finally found out the truth – S waves pass the inner core but disappears when it hits the outer core. This means that the center of the Earth is liquid and not solid.
