Fractals are commonly used in computer graphics to generate terrain. On natural way to proceed is start with a tetrahedron and then determine the mid-point of each edge:

Each mid-point is then displaced by a "randomly selected" value and the tetrahedron is subdivided. For example:

This process then continues until the desired mesh is obtained.

Downloads:
Image (A Collection of Terrains)
Image (Another Collection of Terrains)
Video (640x480, WMV, 6Mb)
Video (640x480, AVI, 14Mb)
Video (320x240, MPG, 4Mb)