Falling sand. A while ago I played a couple of different particle physic simulations based on falling sand. Attempts to shoehorn this idea into a game were, at best, experimental in nature. Most of them were merely sandboxes in the truest sense.
Leave it to PixelJunk, independent creators of original concept games to finally make something worth playing out of these ideas. Currently named 1-4,tho the plan is to come up with a better name. More to the point, the plan is to have you come up with the name. Head over to http://pixeljunk.jp/namingcontest, learn a little about the game, and throw in your hat. Personally, I think “Cymon’s underground rescue/fluid simulation” has a nice ring to it.