A journey of 35,000 meters begins with a single swing

Yeah, the Moho might be a bit much to ask but then so was digging your own basement.

And that's what this PR is.  Open a stairway to z-1 from z=0 just about anywhere (except within about 5 tiles of an existing stairwell; the stair-finding algorithm breaks down a bit then).

It'll take you: construction 5; 7 hours; a pickaxe, shovel, hammer, and axe; and some rope & lumber, so this ain't a cheap operation.  But your fixed base can now have a basement to go with.