Preparing my Sandbox experiment, I realized I never posted these maps. That’s a bit weird, because I like the concept.
The Impossible Earth – a working title – was once your average fantasy world, with a huge underworld hidden beneath the surface. One day, the underworld collapsed all over the planet, and in the resulting cataclysm most of civilization was destroyed.

The underworld’s caverns were so large that the world’s oceans were drained into them. In fact, this drain is still going on, with one minor ocean remaining on the surface.

The dark gray areas on the surface map are caved-in areas – here the former underworld is exposed to the sky.

The main layout of the underworld – of course countless smaller caves riddle the seemingly solid areas.

And that is how a composite of surface and underworld looks like.
As always the maps have a discussion forum on the Cartographers’ Guild.
I personally love this concept, and I have never heard of anybody else doing this before. I won’t be so bold as to claim this is a completely new idea of course. At any rate, I’d love to hear what you guys think of this concept!
Edit: Work on this map continues.
This is a brilliant concept! You could have tones of fun with this idea. The mythology each civilization created would be fantastic. Some might be suspicious of the 'surface' or 'sky land' while other mad individuals will activly seek to explore the land above. You could have expensive mineral deposits found on the top land.
Another idea would be the "final walk" – I'm thinking Judge Dread. Where a retiring Judge is sent to walk the wastes. However in this case you could have either a knightly order or similar who either clime to the 'surface' to die or as a right of passage.
Out of interest do you have a geological reason why the surface land turned into barren desert? Is it because all of the worlds seas are now very much "below ground" and so it has effected the precipitation patterns/atmosphere in some way?
As I said, great idea and gorgeous maps!
Nate
The World Building School
Hey Nate,
Thanks for dropping by, and for the kind words.
The reason why the surface is a desert is simply that most of the moisture gets trapped in the caves. Yes sections of the world are open, there will be some rivers / lakes on the upper world so it's not all a dead wasteland. Think of, say, Egypt, where there's the fertile Nile valley and lots of dry sand to all sides beyond it.
As the surface world dried up, many of the survivors probably invaded the caverns below to find a new life there.
They and some of the subsurface civilizations will battle it out, and once things calm down – I think the main reason to raid the surface would not be minerals, but probably the left overs of the old civilization – art, information, technology, magic items, whatever.
Yeah there should be many things that can be done with this world. 🙂
Very nice, Nils.
Thanks.