Curiosity has found sedimentary rocks that seem to have been deposited by a lake in Mars' Gale Crater 3.5 billion years ago, but this is where we run into the "faint young Sun paradox." Back then, the Sun was only emitting about 70% the amount of energy it pumps out today. Mars' atmosphere couldn't compensate for that lack of heat, as it didn't have enough carbon dioxide to produce a greenhouse effect able to keep water in a liquid state. That would seem to make it impossible for liquid water to have been on Mars, and yet, the new evidence suggests otherwise.
"We've been particularly struck with the absence of carbonate minerals in sedimentary rock the rover has examined," Thomas Bristow, of NASA's Ames Research Center in Moffett Field, California, said. "It would be really hard to get liquid water even if there were a hundred times more carbon dioxide in the atmosphere than what the mineral evidence in the rock tells us."
It doesn't appear this paradox will be solved anytime soon, but as Curiosity continues to gather data and more missions to Mars are planned, we ought to get closer to learning how the dry, lifeless planet used to be so wet.