You can just use meshes. You could use a voxel system if you want, but you can also just find or create the meshes you need. Voxel systems are best if you need the terrain to be editable at run time.
Sadly, I’m artistically challenged haha, making the caves/openings straight from mesh, may be beyond my capabilities, as I’m not familiar with Blender, Maya, etc.
That first one seems exactly what I’d need, too bad it’s not quite out yet.
I bought Unlimited Terrains a day ago, and it’s pretty much dead, no support and extremely difficult to grasp, at least for me. I may have to see what I can do until that TerrainGen comes out.