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The Eridania basin of southern Mars is believed to have held a sea about 3.7 billion years ago, with seafloor deposits likely resulting from underwater hydrothermal activity. This graphic shows estimated depths of water in that ancient sea.
A recent estimate of the total water volume of the ancient Eridania sea is about 50,000 cubic miles (210,000 cubic kilometers), about nine times the total volume of North America's Great Lakes. The map covers an area about 530 miles (850 kilometers) wide.
The reference bar (Figure 1) indicates color coding of depth, from red, at right, showing depth of about 300 feet (100 meters) to black showing depth more than 10 times that depth. This graphic was included in a 2017 report "Ancient hydrothermal seafloor deposits in Eridania basin on Mars" in Nature Communications.