Inner-Earth

Inner-Earth isn't literally another circular ball of rock inside Earth -- it's actually a gigantic cavern, many miles below the surface, that stretches (in surface terms) from northern South America all the way to northern Europe and the Arctic. Primarily it consists of multiple caverns connected by what's known as the Sunless Ocean -- but of course there might be other parts of the Inner -Earth not directly connected to this area, just waiting to be discovered.

The caverns that make up the Inner-Earth are enormous -- hundreds of miles long, and usually at least a mile from ground to ceiling -- allowing for weather, plenty of room for travel, the expansion of civilisations, and so forth.

Inner-Earth, of course, doesn't receive direct light from the sun or moon. However, countless veins of pure quartz crystal, some of them quite large, honeycomb the rock between the ceiling of Inner-Earth and the surface (where they usually emerge in out-of-the-way and unexplored locations, so no one knows about them). These crystals refract sunlight and moonlight down to Inner-Earth, even giving an approximation of sunrise and sunset.