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