The Material Plane is enormous, and consists of several different layers. On the top half is Belagria, consisting of the Surfacelands and the Underrealms. The surface lands are where most of "life" occurs. Countries bicker over land and resources, empires rise and fall, and life goes on. The Underrealms, on the other hand, are a 200 mile thick crust of stone crisscrossed with tunnels and caves, and filled with mysteries and monsters.
As you travel deeper into the Underrealms, the downward pressure keeping your boots pressed to the ground grows weaker and weaker until you eventually reach Emphathum—a 100 mile stretch of weightlessness, filled with floating stones and absolute darkness.
If you down even further, your weight returns, but in the opposite direction from before. Now, you would be heading up, and would be in the second, lower half of the Material Plane—Phlethageros. Phlethageros is dominated by a Zylogmus Eternus, a massive maze, which eventually ends at the lower surface known as the Wastes—the Phlethagoran counterpart to the Surfacelands.
Emphathum
A region with zero gravity, massive floating rocks similar to asteroids, and almost no life. Few have ever crossed this. It is the space between the Underrealms and Zylogmus Eternus.
Phlethageros
The world on the underside of Belagria. Cruel, corrupted, and horrible, it is a brutal world.
Zylogmus Eternus
Millions of miles of labrythine passages stretching across the entire plane, for the entire 200 miles beneath the Wastes and Emphathum.
The Wastes
The barren, lava covered, poison-gas filled, tornado ridden surface of Phethageros. Nothing gets past this—a barrier between the demons above, and the prisoners of Zylogmus Eternus below.
Stalactites of Geth
Massive, low hanging stalactites constantly pour forth lava from above the Wastes. Hundreds of thousands of demons live in these stalactites.
The Edges of the World
North/South/East/West: if you walk far enough in any of the cardinal direction, eventually you will wrap around the side of the world and into Celestia.
Up: Although it is difficult, by traveling directly up far enough, you eventually reach the Astral Sea, and then the Astral Plane itself.
Down: The material slab continues physically downward through rock into Emphathum, then Phlethageros, then the Wastes. Continuing past the Wastes—off the underside of the slab itself—brings you into Malstaris. Malstaris is both the direct counterpart to the Astral Plane (Astral is up; shadows, Nydus, the Black Sun are down) and the literal destination of physical descent past the Wastes.