Yep, this is pretty much how it works in similar applications (LL might have some wonky proprietary way). The grayscale values of the height map store one height per pixel and are translated into 3D space as a triangular-face regular network of polygons textured as ground. Doing it this way allows for a reasonably crunchy and detailed terrain with very, very little file sizes needed to be streamed to the client. I'd make a wild guess and postulate that we can upload textures that are larger than the heightmap for a single sim.
Switching to true 3D terrain would mandate much fatter sim topography files needing to be streamed to everyone. Instead of, for example, a 256x256 grayscale heightmap representing 65,536 pixels or grid points, a 256x256x256m matrix of terrain points (maybe stored bitwise as terrain/air?) would be 16,777,216 points to stream for each approaching client, plus surface textures. Of course, that's assuming folks would want the max excavation depth to be 256m. It'd prolly be less if implemented, ya?
EDIT:
Well, I guess clientside permanent storage of the ground textures is pretty likely, eliminating that need.
Switching to true 3D terrain would mandate much fatter sim topography files needing to be streamed to everyone. Instead of, for example, a 256x256 grayscale heightmap representing 65,536 pixels or grid points, a 256x256x256m matrix of terrain points (maybe stored bitwise as terrain/air?) would be 16,777,216 points to stream for each approaching client, plus surface textures. Of course, that's assuming folks would want the max excavation depth to be 256m. It'd prolly be less if implemented, ya?
EDIT:
Well, I guess clientside permanent storage of the ground textures is pretty likely, eliminating that need.
However, you don't have to have additional thickness, just the illusion of such, with blank "dead" space outside of the paper thin tunnel and cave walls. as long as the tunnels all connected well, and connected to the surface level well, such as a lava tube coming up in a rock-filled "collapse crater. It would appear that the tunnels and caves were going through solid earth, while they are actually suspended in open space.
If you've ever played EQ or similar games, you can sometime align yourself next to or "in" a wall and look out over empty space at other sections of the caves/tunnels in the distance..