Jason Palmer, reporting for BBC News:
The solid iron core is actually crystalline, surrounded by liquid. But the temperature at which that crystal can form had been a subject of long-running debate. Experiments outlined in Science used X-rays to probe tiny samples of iron at extraordinary pressures to examine how the iron crystals form and melt. Seismic waves captured after earthquakes around the globe can give a great deal of information as to the thickness and density of layers in the Earth, but they give no indication of temperature. That has to be worked out either in computer models that simulate the Earth’s insides, or in the laboratory.
A clever way to measure that which (probably) can never be measured.