Some tips on how to optimize your render times and quality thanks to Oshyan.
Few ideas in this post of how to do caves. dandelO found using a plane object with a crater shader rotated, and Seth uses canyons converging
A shader is a program or set of instructions used in 3D computer graphics to determine the final surface properties of an object or image. This can include arbitrarily complex descriptions of light absorption and diffusion, texture mapping, reflection and refraction, shadowing, surface displacement and post-processing effects. In Terragen 2 shaders are used to construct and modify almost every element of a scene.