It's apparent.
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F10, change the resolution of the render to make it really wide then apply it to a flipped cylinder in the skybox. Easy as that.
if dat werkz den plus rep is always nice
Use my method but get rid of the skydome (change the background color to something that is similar to your mountains and make sure to save your final files to a format that supports alpha channels). Basically, render out one pass with the cube in a normal configuration and then rotate it 45 degrees and do the same thing. Get those renders, label them and put them into some panorama creating software.
People do this all the time for Counter Strike, they basically make a ring with nodraw textures all over it and the moutain texture applied to the inner faces facing the level. For halo you just make like an 8 side cylinder the size of the level, flip the faces and apply your mountain texture. Make sure you set the shader to use alpha transparency.
I was wondering the same thing too actually. I was thinking maybe you could have a bunch of mountains as separate meshes and render them individually, then put them each on individual planes and add them to a regular sky dome.