The Hollow World

The planet of the Known World is not a solid ball like our Earth is. In fact
it's hollow. A tiny sun hangs in the center of the hollow, and vast polar 
openings permit travel between the inner and outer worlds. The interior 
landscape is called the 'Hollow World', a place of acient mystery.

The polar openings leading into the Hollow World are so huge that their 
cruvature is very, very gradual. That, combined with the fact that these
regions are perpetually shrouded in fog, means that people can travelfrom the
outer world to the inner (or vice-versa) without being aware of it.

The interior sun sheds daylight at all times in the Hollow World; night never
falls, and creatures susceptible to sunlight (such as vampires)  are rare
indeed.

The interior world has long been hidden from the outer world- the few 
outer-world explores who have found it have either died there, stayed there 
willingly, or returned without publicizing their discovery. To this day, the
existence of the Hollow World is not generally known in the outer world.