A daveworlder friend posted a link about keyhole gardens (which also got emdot interested). These things capture my imagination, too, especially as I read this morning that Ethiopia is again suffering drought and the attendant crop problems.
A couple videos about it. Neet!
It occurs to me, though that while this (along with other versions of raised garden) is a practical application in places with abundant dirt and rocks, the urban and suburban areas near me do not have abundant dirt and rocks. If I want rocks, I have to buy them. Sometimes I can score free pavers or bricks from people who are redoing theirs. I lust after construction rubble but haven't attempted to collect any yet. Dirt is more difficult. Can't really mine my own yard - no hills or similar, and the back has a septic field I wouldn't want to risk uncovering.
However, one resource we do have in abundance here is discarded furniture. I bet an overturned dresser can serve much the same purpose if you nail the drawer faces in - discarded dressers usually have dysfunctional drawers but the faces generally are intact. If at least one or two drawers are intact, some soil savings could occur as well. I still don't know how to get dirt without buying it, but will keep the eyes and ears open. This is a flat place.