: These structures could have inner and outer walls so they could be half in
: and half out of the ground. If they are completely buried then the outer
: walls could be removed by the engine to save resources. Openings are just
: door frames that you place in the solid walls and they become entrances.
: You can place ledges, ramps, roads and tunnels up to them.

: I'd also have a selection of cave interiors to place in cliffs and under
: ground.

: Maybe we need a canyon digger and we could remake AotCR.

Someone asked a designer at Bungie about that exact idea when Reach was current, and the response was something along the lines of, "well that's interesting, that had crossed my mind, but it would take a lot of work and probably end up glitchy, and we'd have to do ground differently, and flag negative spaces, and mess with textures"... or something like that. I believe it was reposted here on HBO.