The Paraelemental Planes (PWH)

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The planes of Smoke, Magma, Ooze, and Ice lie between the main Elemental Planes. They are realms of duality, in which two forces have combined to make one. Less known and less traveled than the main four planes, they also hold fewer native inhabitants.
Where Air and Fire meet, they produce Smoke. The plane of Smoke is similar to the plane of Air, except that (of course) it's filled with foul smoke instead of pure air. A body can't breathe here without magical assistance—the hot, sooty atmosphere is completely saturated with choking clouds and noxious fumes. Like on the plane of Air, down is relative and beings have to fly in order to get around. Earth pockets are few and far between, so if some barmy's going to build a home here, it'll probably have to be on a large cinder floating in from the plane of Ash. There's little native life here to do any building, except at a place called the Choking Palace, where Ehkahk the Smoldering Duke rules over smoke mephits and other fume-based beasts.
Fire and Earth combine to form a plane even more inhospitable than the plane of Fire. Imagine the plane of Fire with an unstable, semi-liquid surface underneath, constantly churning and shifting—that's the plane of Magma. The efreeti and dao meet here to trade, and a few other creatures of earth and fire dwell on this plane rather than their own. Otherwise, lava mephits are all one's going to find here. A basher named Chilimba claims dominance over the entire plane, and nobody cares enough to challenge his rule.
Earth and Water merge to form the Paraelemental Plane of Ooze, a place as bad as it sounds—mud and slime fill an ocean with no end in any direction. Conditions aren't unlike those on the plane of Water, but the substance here is opaque and thick. Creatures drawn here through vortices drown in the sludge—virtually nothing can survive in the muck save ooze mephits. No known cities or palaces exist, even in the few pockets of clear water or fresh air. The only "use" of this place comes from its function as a prison. If a wizard's got a mean streak, she's likely to dump her enemies here. If she's really cruel, she'll send 'em here with some magic allowing them to breathe but without anything to eat or drink. Such banishments have lead to this plane's pseudonym, the House of Chambered Madness. The current chant is that there's a secret way into Sigil from this plane—one even the Lady of Pain doesn't know about.
Water and Air form the plane of Ice, which has a navigable surface and breathable air—and nothing else but ice and snow. Endless glaciers, ice flows, snow fields, and frozen seas make up the landscape, and digging down into the ice won't reveal anything more underneath. Occasional pockets of air or earth mar the landscape, but mostly the ice just goes on forever. At the heart of the plane, Cyronax rules over all ice elementals and mephits, hoping to extend his influence even onto other planes. Rumors abound, however, of a race of powerful beings that even Cyronax knows nothing about, frozen forever under tons of ice, waiting to be released from their cold prison.
Tarsheva Recommends: The plane of Ice produces a rare resource called eternal ice or unmelting crystal. Simply put, it's ice that won't melt at any temperature. It is likewise extremely hard, almost unbreakable. Weapons and other items made of this substance are very valuable, although quite rare because of the scarcity of the ice and the difficulty in obtaining it and working it into a usable shape.