Opportunity has proven itself to be the energizer rabbit of rovers. Having been on Mars for nearly 10 years it just keeps going and going. Data from Opportunity was instrumental in the development of the Catastrophic Model of Martian Geology. One aspect of this model is that the Meridiani Planum region of Mars was flooded by fresh water; most likely an eruption from underground; that would have become highly acidic as the result of heavy volcanic activity.
Recently Opportunity found evidence of clay in a rock called Esperance. Because clay forms in PH neutral is evidence that a neutral fresh water environment preceded the acidic environment of Meridiani Planum as predicted by the Catastrophic Model of Martian Geology. This is further confirmation of this young Mars (young Earth as well) model of Martian geology.