Here's something I've thought of in the past, but it just occurred to me again with a commercial I saw on tv.  It has always been easy to add objects on the ground, but very hard to dig down, to show holes, trenches (or the commercial was showing the 9/11 memorial.  It would be interesting if there was a way to do that.

Very interesting question as it raises another issue to which Asobo has not brought a clear indication : how the natural relief is generated ? Mesh or extrusion computed from sat imagery (or both) ?. The second solution had the power to be more precise than the latter.

On the question itself, it seems to me that it is basic mesh work. BTW, in the present state of things, adding a small, irregularly shaped island on a lake is not exactly easy, in my experience. And digging a regular hole should’t be difficult but I haven’t tried.