Mundus the term for the entire world on which the continent of Vosca is situated, and is one of the Tattered Realms, along with Void. The shape of the world is unknown. The surface is populated by men and the Din. Below the surface is the Hollow, which is connected to the tunnels of the Dwarves.
There are islands floating in the skies above the surface, though they are too high to reach. Sky beasts can be seen traveling among them. There are also "fleeting stars that pass predictably through the night sky", probably planets or satellites from previous ages.
Mundus has seen a lot of history, and many great civilizations have risen, based around plentiful and useful resources that they then exhausted. They created great wonders in their time but most of it has, as high-tech stuff tends to, decayed over time and become worthless. The shelf life of advanced technology is not terribly long. These civilizations did not (usually) expect to be destroyed or to collapse, and didn't build most of their stuff to last.
Those things that have survived were intentionally built to survive, meant to endure thousands of years. Many speculate that the Black Obelisks are the product of ancient civilizations, and of course legends abound about the floating islands, or the fleeting stars that pass predictably through the night sky.