A Mars mapping project at the Tucson-based Planetary Science Institute concludes that the 2,000 km diameter and 8 km deep Hellas impact basin [right, credit NASA] contains sediments laid down by an ancient sea that filled the body.
Dr. Leslie Bleamaster published the 1:1 million scale map using data from an array NASA' Mars spacecraft according to a report on Mars Daily (who knew that there is a daily Mars newspaper?).
The lakes are dated at 3.5 to 4.5 billion years old.