The Mars Ocean Hypothesis is a hypothesis that nearly a third of the surface of Mars was once covered by an ocean earlier in its history. The ocean, which is called Oceanus Borealis, would have filled the Vastitas Borealis basin in the northern hemisphere. The Vastitas Borealis is 4–5 km (2.5–3 miles) miles below the planetary elevation. Oceanus Borealis dried up 3.8 billion years ago. Early Mars would need a warmer climate and thicker atmosphere to let liquid water stay at the surface.