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.[2][3][4] This primordial ocean, dubbed Paleo-Ocean[1] or Oceanus Borealis (/oʊˈsiːənəs ˌbɒriˈælɪs/ oh-SEE-ə-nəs BORR-ee-AL-iss),[5] would have filled the basin Vastitas Borealis in the northern hemisphere, a region which 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.[6][7][8] Early Mars would have required a denser atmosphere and warmer climate to allow liquid water to remain at the surface.[9][10][11][12]