With two of my friends, we develop, since 2015, a program on Unity3D capable of making realistic procedural worlds in 3D.

Some features at present:

• Real sized environments (1:1 scale terrains)
• Progressive level of detail system
• Mountainous area procedural algorithm
• Procedural material and vegetation placement trying to mimick natural processes
• Realistically scaled volumetric cloud coverage and weather system (In development)
• Terrain editing, roads, dirt paths, rivers, lakes (In development)

And some technologies used:

• Multithread programming, C#
• Shaders programming, CG
• DEM Data Manipulation
• GPU Computing using a C++ DLL, DirectCompute/HLSL
• Raymarching

If you want more informations, explanations, images and videos, you can look at our dev blog and these screenshots!