Hopefully you have seen it in game. The ground there can be destroyed as if it were voxels. But it does not seem to use Roblox's in-built Terrain System because it uses different Materials and every voxel (is it voxels?) is a perfect cube shape.
I do not think it is made of Parts because for a map that size it would have to be 2000x2000 cubes which is 4 million Parts and that is by far more than any game can handle. I am new to Roblox but I am a mid-level programmer in other languages, but not with games.