You've got me thinking....... I'm going to try this in Terragen. I'll create a simple terrain file. Save it, I'll press combine in the Terrain box, then hit copy. Invert one layer, then click combine. Have you googled that question? If my idea fall flat on it's face, I'm sure someone will know somewhere. I remember years ago, mid 2000's, I created a cave, but that was by rendering two landscaped and flipping one over the other as someone mentioned.