Gypsum Vein = water
Y'all might have seen the story a few weeks back, but I don't recall it showing up on the Zone. Recently, that intrepid rover Opportunity studied a neat little outcrop:
Tests reveal that it is gypsum (which requires water to occur) and that the rocks are insitu. That's where the gypsum formed - deposited by ground water in a fracture!
Could be Oppy's biggest discovery to date.
Here's the link: NASA Press Release