If you mean tunnel as in actually traveling through the ground, you could make a second terrain and place that over the dug out hole. I don't know how this would work with the AI seeder though. If you mean actual roofs for like houses, I believe those work similarly to wall placement. There's a tutorial video about it
I just mean like..a quick cave on the side of a mountain or big enough indent to look like one. For some reason you can only cut terrain oer the entire vertical axis, but can't do it per pixel independent of that axis.
I just want to select a portion of terrain and get rid of it. Can't figure it out, if it's possible.
