The Quasielemental Planes (PWH)

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The raw elements intersect with the pure energies of the Positive or Negative Energy Planes to form the eight Quasielemental Planes, or quasiplanes. Air, Earth, Fire, and Water and positive energy mix to create the planes of Lightning, Mineral, Radiance, and Steam, respectively. Similarly, Air, Earth, Fire, and Water combine with negative energy to form the planes of Vacuum, Dust, Ash, and Salt. Some are beautiful, some are horrible, and others are just deadly. All are dangerous in one way or another to visitors. They're also not commonly frequented by planar travelers, although the Doomguard have built fortresses within each of the quasiplanes influenced by the Negative Energy Plane. Likewise, mysterious towers stand on all four of the positive quasiplanes, but their unknown builders have apparently long gone.
The plane of Lightning is often called the plane of Storms or the Vengeful Land by planewalkers and poets. It's like the plane of Air in every way, except that there's always an electrical storm crashing down upon a body's head. (Rain rarely accompanies the storms.) All visitors and objects have crackling glows about them, and metal automatically attracts lightning bolts, accompanied by terrible thunder. The mysterious Tower of Storms is the only known structure here, but not even the native lightning mephits and shockers know who lives inside. 
Planewalkers sometimes refer to the plane of Mineral as the treasure-trove of the multiverse, but only to clueless rubes who don't know the dark of it. Sure, the plane's filled with gems, gold, silver, and other treasures waiting to be mined, but the place is extremely dangerous and well guarded. Native creatures like xorn, pech, and dao don't like leatberheads pouring onto their plane and carting off the elemental stuff it comprises. Getting through and around it poses the same challenges as does the plane of Earth, except everything has sharp edges that cut like knives. Much worse, however, is that everything here fossilizes at an incredible rate, and sods remaining here too long turn to stone. Although the stone's valuable, that's small consolation to a petrified berk. Finally, Mineral hosts another of those enigmatic fortresses; it's said that forging done at the Tower of Lead is far superior to that performed anywhere else on the planes.
Cascading light, brilliant color, and mind-numbing illumination fill the plane of Radiance. Without protection, a traveler will go blind, and quickly too. It's both beautiful and horrible at once. Radiance is as empty as the plane of Air, but as hot as the plane of Fire; only radiant mephits live on this plane of heat, light, and color. Near the edge of the Positive Energy Plane stands the blue-lit palace called the Heart of Light, where great healings are possible.
Much cooler is the Quasielemental Plane of Steam, a chilly, misty place so thick with clammy vapor that a body runs the risk of drowning in it. Many bashers compare this plane to the plane of Air for its relative emptiness—other than the mist and the steam mephits, there's little of interest except for the Tower of Ice, which bas some sort of magical effect over potions and other mixtures blended here.
Vacuum is essentially a void of absolute nothingness—nothing to breathe, nothing to stand on, nothing to swim or fly through. Nothing. At its edge, the Doomguard's Citadel Exhalus, the Portal of the Last Breath, drifts anchored by a thread between Vacuum and Negative Energy. If a cutter can come up with a way to stay warm and do without breath (since there's nothing here to convert to breathable atmosphere), he can travel by sheer force of will, his mind taking him where he wants to go. Rumors say that beings of pure thought, without mass or energy, dwell here, and possibly travel out onto the other planes. No one knows for sure—how could they?
Travelers on the plane of Dust find themselves slowly disintegrating, their very forms breaking up into the dust of this place. Despite this, Dust has a number of native inhabitants: dune stalkers, dust mephits, sandlings, and sandmen. Here the Doomguard have built their most popular tower, Citadel Alluvius, its magic protecting those within from the harshest aspects of the plane. The barmiest (or most fanatic) of those addle-coves head out into the plane deliberately to disintegrate into nothingness. Maybe it's their idea of the perfect entropic death.
The plane of Ash has fewer inhabitants than Dust and is as bad as Fire, for a traveler can't breathe here, and the cinders drain a body's warmth at every turn. Not many come here, for who's going to swim through an endless sea of cinder and ash on purpose? Well, the Doomguard maintain the Crumbling Citadel—the fortress and the surrounding plane suit their moods and philosophies. Similarly, a powerful lich named Vecna "lives" in a huge stronghold here. Chant is that once-living prisoners of Vecna are transformed into horrible undead aberrations unlike those found anywhere else, their new forms a result of the fortress's proximity to the Negative Energy Plane.
The plane of Salt is an oft-forgotten place of dry, moisture-leeching crystalline crust. Portions of the place are more liquid than others, but much of the plane of Salt is dry and parched, causing a body to shrivel and desiccate. Except for salt mephits, nothing can live here. A fortress carved of salt itself, Citadel Sealt (yet another Doomguard palace), can be found in a hollowed-out place within the most solid crystalline portion of the plane.
Tarsheva Recommends: The plane of Ash is said to be the home of an exiled efreet lord who rules a small city within its wastes. Despite the fire king's evil temperament, travelers with considerable jink can find shelter, supplies, and aid of all kinds.