Pickover’s 1998 article “Vacation on Mars” describes a simple algorithm for auto-generating mountainous terrain data.
The simple idea is to stamp circles onto a 2D array, while moving the stamp position to follow a random walk pattern. A random walk pattern means that, at each iteration, the position moves by a small, random amount in some random direction.
The Mars library can be used to generate the array data and, ultimately, a PLY file, which can be loaded into a modeling program such as Blender. The Mars library is free software available at the Hackage repository, while Blender is a free software 3D modeling program.