Despite being relatively close to us the inside of the Earth, and particularly the core of the Earth, is difficult to investigate. Primarily because we can't just look at it - and the deepest mines or boreholes are only 10km deep which is tiny compared to the 6,000km that is the Earth's radius. So everything needs to be logically deduced from the readings that we can take.
Absolute zero, or 0°K is the minimum possible temperature, and there has been a race of sorts over the last couple of centuries to reach that temperature in the laboratory. The experts discussing it on In Our Time were Simon Schaffer (University of Cambridge), Stephen Blundell (University of Oxford) and Nicola Wilkin (University of Birmingham).