What are vastitas borealis?

Vastitas Borealis

Vastitas Borealis (Latin, 'northern waste' ) is the largest lowland region of Mars. It is in the northerly latitudes of the planet and encircles the northern polar region. Vastitas Borealis is often simply referred to as the northern plains or northern lowlands of Mars. The plains lie 4–5 km below the mean radius of the planet. To the north lies Planum Boreum.

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Some articles on vastitas borealis:

Hesperian Chronology and Stratigraphy - Boundaries and Subdivisions
... boundary of the Hesperian with the younger Amazonian System is defined as the base of the Vastitas Borealis Formation (pictured right) ... The Vastitas Borealis is a vast, low-lying plain that covers much of the northern hemisphere of Mars ... Another interpretation of the Vastitas Borealis Formation is that it consists of lava flows ...
Water On Mars - Extinct Water Bodies - Mars Ocean Hypothesis
... The Mars Ocean Hypothesis proposes that the Vastitas Borealis basin was the site of an ocean of liquid water at least once, and presents evidence that nearly a third of the surface of Mars was covered by a ... This ocean, dubbed Oceanus Borealis, would have filled the Vastitas Borealis basin in the northern hemisphere, a region which lies 4–5 km (2.5–3 miles) below the mean planetary elevation ... The lower, the 'Deuteronilus', follows the Vastitas Borealis formation ...