Digipen - which is a pen into which students are herded and then forced to make digital entertainment for their sinister cowled overlords - produces some excellent games. 8-bittest among these is the superb Dig-N-Rig, in which you control a mining robot who can dig and then hoover up the minerals he has discovered. A familiar motif, but the rig part is more interesting: you have to construct a series of horizontal and vertical conveyors to send your mineral loot back up your factory-HQ on the surface. Mining therefore becomes self-perpetuating, as you have to pay for bits and pieces to expand the rig, and to upgrade your robot with things like better wifi, so he can dig deeper and further from his base.
Dig-N-Rig is, as games about digging and mining tend to be, rather compulsive. But combine the building, the surprising complexity, and the glorious 8-bit 2D execution, and you have something that is going to eat time like some kind of desktop-based black hole. I blame Indiegames for digging this one up, now you can too.