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The Mars ocean theory states that nearly a third of the surface of Mars was covered by an ocean of liquid water early in the planet's geologic history. This primordial ocean, dubbed Paleo-Ocean or Oceanus Borealis (/oʊˈsiːənəs ˌbɒriˈælɪs/ oh-SEE-ə-nəs BORR-ee-AL-iss), would have filled the basin Vastitas Borealis in the northern hemisphere, a region that lies 4–5 km (2.5–3 miles) below the mean planetary elevation, at a time period of approximately 4.1–3.8 billion years ago. Evidence for this ocean includes geographic features resembling ancient shorelines, and the chemical properties of the Martian soil and atmosphere
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