With all the cave talk and having spent couple of weeks thinking about it and playing with the terrain I hit upon the idea of how to get tunnels at least without too much effort.



Caves is simply a no no as it will add to the polygon count far too much, which would result in some thing that might look good but ultimately unplayable due to high polygon count.



Entity based cave is still the best option for this.However with that said I still think it will be reasonably possible to create small sections of tunnels few meters in length.



There is two ways to go about it, GCG option, having terrain shaping blocks that would carve into the terrain.This would still add to the polygon count.But is ideal for a small section of a tunnel, think of some thing like a train tunnel. With CSG options you would be able to carve into a vertical cliff and create an Open tunnel for example.



The other option is using the tunnel flag, where you would create for example a train tunnel shape and curved as you require, and simply paint over the tunnel, and force the terrain edges to stop where the entity starts, some thing like the auto leveling feature of entity snapping.You could create a cave theoretically using the above options, however performance would suffer, but is ideal for small sections round or squared shaped, the more basic the better.It wouldn't cost that much in poly count, it would also open the door for more terrain shaping options, without the need for additional hard coded painting tools.



You can even take it one step further with the last option, and have the inner polygons of the terrain not render at all because you would have a entity blocking the faces of the terrain inside the tunnels any ways



The only problem with CSG, is of course uv maps being messed up.With CSG the options it would literally be limitless.



But you have to ultimately accept, that it would be basic, to keep polygon count down.