The known world appears flat to observers. Whether this is it's true shape is fiercely debated by sages and philosophers.
The general consensus on the known universe is that is made up of a series of elemental and material planes, layered vertically one upon the other and coexisting with the spiritual planes.
These planes are arranged not by density (relative heaviness) but by temperature, from the hottest at the bottom, to the coldest at the top.
The Plane of Fire
The bottom-most plane is the plane of fire. Consisting of the of the lightest of elements, it is the home of elemental creatures and fiery demons.
The Underworld
Between the planes of fire and earth lie the realms of the dead: Haidesium, Mag Mell, and Hel.
Here you find the souls of the forsaken and the damned, tormented by fiends and ruled over by Agesilaos, Arawn Annwn, or Hölle.
The Plane of Earth
Comprising of the heaviest of elements, the material plane of earth covers the plane of fire, keeping it's contents from erupting into the rest of the planes — most of the time.
The Plane of Water
Made up of the second heaviest element, the plane of water rests on top of the plane of earth.
The Material Planes
Where sections the elemental earth is forced by the fire beneath it to rise above the top of the plane of water and into the plane of air, the four elements combine to create a material plane. Although it is conjectured that many may exist, the only one known to explorers and scholars alike is the prime material plane, home to Terramunde.
The Plane of Air
Composed of the lightest of elements, the plane of air is sandwiched between the heavens and the plane of fire.
While all heat and flame in the material planes originate from the plane of fire, light and electricity emenate from the far reaches of the plane of air.
The Heavens
Floating on top of the plane of air the heavens: Olympus, Andowvyn, and Walhalla. These realms are inhabited by gods and celestials, and receive the souls of the dead (at least those not doomed to the underworld).
Above the heavens lies the endless void, and possibly other universes.
The Ethereal Plane
The ethereal plane overlaps all the material planes and borders the elemental planes. Within the dimly lit mists of the ether, visibility is limited to 60 feet.
The material and elemental planes are visible from the ethereal plane, but appear muted and indistinct, their colors blurring into each other and edges turning fuzzy. Conversely, The ethereal plane is usually invisible to those on other planes.
Normally, creatures on the ethereal plane cannot attack creatures on the material and elemental planes, and vice versa.
The Ethereal Plane is mostly empty of structures and impediments. However, the plane has its own inhabitants. Some of these are other ethereal travelers, but the ghosts found here pose a particular peril to those who walk the fog.
The Astral Plane
An endless expanse of clear silvery sky, the astral plane overlaps all the other planes, material and elemental, as well as the heavens and the underworld. This allows travel between all of the planes.
Gods, celestials, and even some fiends move freely in and out of the astral plane, whereas mortals must use the strongest of magics to cross it's borders.