By observing how seismic waves are absorbed and refracted, scientist have been able to work out where the properties of the earth change dramatically. our understanding of the internal structures of the earth and the size of its core is based on these observations.
basically:
size of core
structure of the earth I.e. the layers of the earth
and I think the last one is how the internal structure is changing? not sure on the last one but its still a solid guess.
