What's a skybox?
In Second Life, a skybox is exactly what it sounds like: a building in the sky.
Skyboxes are as varied in appearance as ground structures: they range from simple work platforms which aren't technically boxes (for building, photography, and other focused activities) to elaborate houses (like sky castles!) and even whole malls. Also, places that feel natural "in the air", like space stations, could also be considered skyboxes.
How can I get a skybox?
You can, as with just about everything else in Second Life, build your own. Here's a video tutorial showing how to place objects in the sky:
How high can a skybox be?
As shown in the limits list, the build height is 4,096m. However, the base of even a very high skybox will likely be placed below that (for example, 4,000m), allowing for room to be built upwards in case the owner wants to add an extra floor or other expansions.
Since avatars can fly in Second Life and placing objects in the air isn't affected by gravity (unless you choose to enable physics on them), making use of vertical space is very popular for several reasons, including:
- Privacy - It's less likely for both text and voice chat to be heard when it's hard to get to where you are. (Although IMs, instant messages, can be a better option since they can't be snooped on by proximity.) New Residents and casual passerby are less likely to look thousands of meters in the air. Part of this is because there's a built-in fly height limit (also shown on the limits list), which can be bypassed with a scripted "flight assist" gadget, but is otherwise a default restriction.
- Performance - What's in the sky is a lot less dense than what's on the ground. When less things need to be rendered, performance improves. Also, lack of visual clutter can help someone focus on a creative task, not unlike how distraction-free text editors have become popular.
- Immersion - Some Residents have figured out clever ways to create the illusion of a self-contained environment by putting a building inside a textured, giant sphere. This obviously doesn't work on the ground, where the terrain and water are visible. A variation on this are skyboxes that have textures on the windows that look like city landscapes and other "fake" environments to better control the experience.
"Skybox" in other 3D environments commonly means a simple construct to simulate skies.