A: by Sean O'Keefe, NASA Administrator:
We think the Mars Exploration Rover Opportunity is in the vicinity of what was once a shoreline of a salty sea on the red planet. If we are correct in our interpretation, this was once a habitable environment. It's a salt flat.

These are the kinds of environments that are suitable for life on Earth, but we don't know if that was the case on Mars. We have not found any evidence of past life on Mars, but we will continue our efforts to gather data, and maybe eventually answer that intriguing question.