In the late 1600s, English astronomer Edmond Halley presented a model of Earth’s structure. Halley originally claimed that Earth was composed of an outer rocky shell and an inner rocky sphere, separated by a wide gap filled with glowing air. He thought that Earth had two sets of magnetic poles: one permanent set that originated on the surface, and another set that originated in the inner sphere. Halley believed that the inner set of magnetic poles moved over time, because the inner sphere rotated slightly slower than the outer shell. He also thought that the inner sphere was just as habitable as the surface. Halley knew that his model was hypothetical and might not be completely correct, but he thought that it could explain some important observations, calculations, and inferences about Earth’s properties. His model was based on information and evidence available at the time, but it was also based on some nonscientific beliefs that many people at the time accepted. The idea that Earth is hollow was not new—many people thought that there were vast caverns that extended deep into Earth’s interior.

Suppose that you know very little about Earth’s interior and want to find out how accurate Halley’s model is. How would you go about evaluating the validity of the model? What sort of evidence would you need to collect and how could you collect it?