See this post by Skerples because it's really cool.
Also this flash game.
Basically my first introduction to fantasy (Beyond a book of Greek Myths my mother read to me,) was the Dark Elf trilogy by R.A. Salvatore, so I love the Underdark as a concept.
But what about the Overbright?
First off, put on your cartoon logic hat. You can walk on clouds, mine them and juice them with your hands for water. Cloud-soil can take seeds like dirt and grow vines and trees. There are grazing herds of pegasi in elysian cloud meadows miles above the land's surface. The centers of clouds become denser, solidifying into striated marble, which is just as heavy as the land-bound stuff, but hangs in the air all the same.
The Overbright is Isolated yet interconnected. Islands drift together, conjoin or split into smaller cloud islands. Birds and floating souls can cross easily, but clod-footed ground-dwellers need lighter-than-air sky canoes and pegasi driven chariots to get around. This leads to extreme isolation- the denizens of most islands seldom leave, but they are not in danger of starving or stagnating, since migratory birds and sky-bound nomads bring trade and food. Sunlight is abundant in the Overbright, making agriculture and photosynthesis a viable form of subsistence. A good many wizards travel to the Overbright to build their towers- the solitude works wonders for their research.
But don't expect the Overbright to be all sunshine and rainbows, though
those certainly do figure in. Just because it's in the sky doesn't mean
that it's a paradise. You think the underdark invented danger? The Night
Below was a crack in the burning Earth when the Overbright was a storm
as tall as the horizon, formed from the oceans turned to steam hotter
than flame. It's like they say; as above, so below.
The interplay of distance and gravity have a strong affect on Overbright navigation. You can see the lands below, but going down there is almost impossible, since doing so would break every bone in your body. Therefore it's fun to play with that "so close, so far" relationship. Imagine mile long fishing lines reeling up catches from oceans below. The peaks of great mountains brush past as sky-goats disembark, having traveled the whole of the world atop their temporary cloud pasture.
The Overbright is Formless and Ancient. The shifting nature of cloud-islands means that it is impossible to chart the distances between them. Imagine trying to map a three-dimensional ocean of ice bergs that keep smashing into each other. Individual mountain sized clouds become shifting labyrinths, the surfaces on the outside remain, but the geography churns with the wind. All the same, the contents of the islands are ancient.
Due to the difficulty of getting around, when an island becomes abandoned it could be centuries before it is discovered by an explorer. There are ancient crystal sarcophagi held suspended at the peaks of white cloud mountains, marble coliseums of the cyclopes manned by the floating spirits of their long dead slaves, titanic forests held within the hollow pillars of the sky; all of these places untread by mortal feet in centuries.
This makes the Overbright genuinely alien. There are creatures up there that have been extinct for millennia on the surface. There are creatures made for grand cosmic plans for the surface that never panned out: the work of dead gods who never returned home to their private skybound workshops. There are ancient empires and hidden sky kingdoms and wild offshoot tribes of people that genuinely don't know that there's a whole world down there, just like most people don't know that there's a tyrannical dystopia in "that cloud over there that looks like a shoe."
Metal is rare, any bronze or steel in the Overbright came from the surface at some point. Without magma, the largest cloud islands instead churn with unshed lightning, which boils cloudsoil into metamorphic sky-marble, or crystallizes it into igneous fulgurite. Most material crafting in the Overbright must therefore be done with bone, cloudstone, and wood.
The Overbright is Big. It stretches from about a mile away from the surface to the edge of space. That's a whole lot of sky.
So how do I get there?
1. Climbing. Tall enough towers, giant beanstalks, mountains.
2. Divine Benediction. Sometimes the angels painted on chapel ceilings reach down and pull you up.
3. Tornadoes. If you survive getting sucked up into the sky, you might just realize you're not in Kansas anymore.
4. Invitation. Sometimes, somebody lowers a rope. If they're nice, it's got a basket big enough for you and your friends attached.
5. Fog. A fog bank is just a cloud that fell down from the Overbright. Sometimes they rise back up with you on them, sometimes you can find a staircase made out of cloudstuff.
6. Riding something with wings. Dragons, Megaravens, helicopters. If it can fly, it can probably land you on a sky island.
