It's just a dirt terrain with grass blocks on the top. (which are just dirt blocks with green tops.)
I sampled a Unity terrain's heightmap, and constructed a voxel terrain based on it, and divided it into chunks, whose meshes then were combined.
Sadly the generated mipmaps are bad, the mipmap change is easily noticeable: first the shade of green changes, then it fully becomes dirt colored.
The problem appears to be that Unity generates mip maps from your texture without considering the "cell borders" in your textures (how could it at the texture importing stage when it does not yet know how you are going to set up your UV maps?). So the brown parts of the textures bleed into the green parts.
A quick and dirty solution would be to turn off mip map generation, but that will likely result in a performance penalty (and in a scene like that you need any performance you can get) and other kinds of visual artifacts.
So a better solution could be to use three separate texture images - one for the sides of the cubes, one for the top and one for the bottom. That will prevent the mipmap generation algorithm from mixing these textures.
