From the article "Underground" in GURPS All-Star Jam 2004

The astronomer Edmund Halley in 1692 suggested that the Earth consisted of 5 shells, each with its own atmosphere & luminosity.

The five shells could be (outermost first)

1. Late 17th century Earth, with some clockpunk and alchemy

2. Pleistocene creatures, including primitive men

3. Cenozoic creatures, including beast-men

4. Mezozoic creatures, including reptile-men

5. Paleozoic creatures, including frog-men

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