http://science.nasa.gov/headlines/y2009/02dec_troy.htm?list1357767

Spirit has been stuck in one place for the last 6 months which would be bad except for where Spirit got stuck.  It got stuck half in an ancient sulfur/steam vent and half out of it.  They have found a treasure trove of information due to the stuck event.

"And most amazingly, the boundary between the sulfate-rich soil and the soil with just the generic concentration of sulfates runs right down the middle of the stranded rover. Spirit is lodged on the edge of a crater -- sitting astride the boundary!"