The centre of the Earth is thought to consist of a solid inner core and a molten outer core. Both are made up mainly of iron but also contain small amounts of nickel and possibly oxygen. Our knowledge of the core comes from a variety of indirect measurements that include the study of seismic waves and earthquakes, the analysis of meteorites, and high-pressure laboratory experiments.

Stevenson proposes sending down a small “grapefruit-sized” probe, placed in a large volume of liquid-iron alloy. The mass of iron contained in the volume would need to be about 108 kg, which is equivalent to the amount of iron produced by the world’s foundries in about an hour. The probe would travel at about 5 metres per second along a crack under the influence of gravity, and would reach the centre in about a week. It would be made of an alloy with a high melting point and would include instruments to measure temperature, electrical conductivity and the abundance of various elements.

The signal sent out by the probe could, in principle, be detected by the Laser Interferometer Gravitational Wave Observatory (LIGO) - a facility built to detect gravitational waves. However, LIGO is currently isolated from vibrations in the ground and could not be sensitive to both gravitational waves and signals from the probe at the same time. Stevenson believes that his proposal is “modest” compared to the space programme and may appear unrealistic only because so little effort has been devoted to such direct measurements.