Outerra is very impressive 3D planetary engine. It uses fractals, algorithms and a whole lot of maths to let you create everything from an orbital view of a virtual planet down to a blade of grass. It has an integrated vehicle physics engine too.
So, what would happen is someone used it to recreate Middle-Earth? That’s exactly what’s going on with ME-DEM. Here’s a tour.
This is not a finished piece and worrying the site was last updated in Feburary 2013. Nevertheless, the potential is clear and the sheer scope of the project is incredible.