The Inner Planes (PWH)

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The elements form the building blocks of the universe. They make up all that is, creating a foundation for all matter and energy. The Elemental Planes represent form and essence, but hold little thought and no ideology or high concepts. No philosophies bind these planes, and unlike the Outer Planes, they represent no aspects of morality or ethics. They simply are, representing the materials that comprise the Prime.
Surviving on the Inner Planes is usually more of a challenge than on most of the Outer Planes, but certain means (usually magical) make it possible. There's nothing hospitable about a plane formed only of fire, but at least it's predictable—a body knows what to expect on the Inner Planes. Since civilization in any form is rarer here than on the Outer Planes, a planewalker should make sure she brings everything she needs with her.
Each Elemental Plane is made up almost entirely of its base element. The key here, 'course, is almost. Some folks call it slippage, some call it seepage, but the fact is that sometimes bits and bubbles of one element make their way onto the plane of another. For whatever reason, the Inner Planes're full of leaks. Once they enter another plane, these elemental bits—or pockets—are subject to the conditions of that plane. This means that water seeping onto the plane of Fire becomes steam, and fire pockets on the plane of Water simply create hot water. Pockets of earth and air are harder to consume, and so create convenient places for planewalkers to breathe and rest. The Paraelemental and Quasielemental Planes also leak into each other as well as the four base planes.
The Inner Planes don't have layers, but each is infinite in size and scope in its own right. Nevertheless, it is possible to physically travel from one Elemental Plane to another. How? Don't ask. A graybeard'll give an earful of explanations and a body still won't know—mostly 'cause the graybeard really doesn't know either. He's bluffing. Some say that no mortal can really understand how the planes work, and nobody's ever proved them wrong. The point is, these infinite planes really do have physical borders that a body can cross over. Interestingly, this isn't where the aforementioned leaks are, since most of the time, the elemental pockets are not comprised of elements from bordering planes.
These border areas can be almost impossible to find without a native guide or magic. (The elemental rift spell does the job nicely, although it's not limited to taking a basher to only bordering planes.) A canny cutter'll notice that as she travels through one plane toward another, the elemental material of the first plane begins to subtly change to the element of the second. For example, a traveler journeying from the plane of Earth to the plane of Ooze (assuming she's got a guide or knows where to go) notices that the earth and stone begin to get more fluid and more slimy as she heads in that direction. If a berk's on her own, though, trying to get from one inner Plane to another, her best bet's to look for one of the extremely rare inter-elemental vortices that'll pop her right into another plane.
* The Elemental Plane of Air
* The Elemental Plane of Earth
* The Elemental Plane of Fire
* The Elemental Plane of Water
* The Paraelemental Planes
* The Quasielemental Planes
* The Positive and Negative Energy Planes