The black footprint of Ontario Lacus lies within a region that once contained a much larger ancient southern sea.
This image was obtained by Cassini's radar instrument on several flybys inlcluding those on December 21, 2008 (T49 flyby), July and January 12, 2010 (T65 flyby). Several images have been stitched together. By analyzing these images, scientists estimate the ancient sea was possibly as large as 475 by 280 kilometers across and likely less than a few hundred meters deep. Ontario Lacus is about 80 by 235 kilometers across, and probably at least 10 meters deep at its center. Seas may have covered large parts of the southern hemisphere less than 50,000 years ago.
NASA / JPL
Ontario Lacus, Titan, from Cassini RADAR (annotated)
The red outline shows the putative boundary of an ancient, much larger sea, of which Ontario Lacus is only a remnant.
Titan, Saturn's largest moon, is the only place besides Earth in our solar system that hosts large open bodies of liquid. At the cold temperatures of Titan, about 94 kelvins, that liquid is not water, but methane and ethane. Over one hundred lakes and three seas are seen at the north pole of Titan, while the south pole only has a few small lakes. Scientists have suggested that cycles analogous to Croll-Milankovich cycles on Earth cause long-term cyclic transfer of liquid hydrocarbons from pole to pole, with the north pole now containing the bulk of the liquids. Less than 50,000 years ago, the cycle would have been reversed, suggesting that the south polar region should contain remnants of southern seas.