[ODE] solid terrain representations that are still tunnelable?

[Megan Fox]

Given my spec, I need a heightmap terrain that I can drop down through
(caves, crevaces and the like), but I'm still not wild about the idea
of a trimesh paper-thin collider for the entire thing.

Given this, I'm wondering if anyone has tried something like a layer
of thin boxes, one per heightmap grid, oriented to the grid's normal
and sized to cover the grid entirely.  Seemingly, this would give you
a terrain representation that had the advantages of a solid geom,
while still allowing for the creation of holes through it and the
existence of geometry under it.  You of course wouldn't actually have
a separate geom per grid, this is just how you would treat the
collision calculations.

(this as opposed to a "anything under the heightmap is penetrating" approach)

The one problem I can see with it would be a possibility of strange
collisions on extreme external points in the heightmap terrain, though
it would depend largely on the depth of the box representation.

-Megan Fox