(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, northern lowlands or the North polar erg of Mars. The plains lie 4–5 km below the mean radius of the planet, and is centered at . A small part of Vastitas Borealis lies in the Ismenius Lacus quadrangle. The region was named by Eugene Antoniadi, who noted the distinct albedo feature of the Northern plains in his book La Planète Mars (1930). The name was officially adopted by the International Astronomical Union in 1973. Although it is not an officially recognized feature, the North Polar Basin makes up most of the lowlands in the Northern Hemisphere of Mars. As a result, Vastitas Borealis lies within the North Polar Basin, while Utopia Planitia, another very large basin, is adjacent to it. Some scientists have speculated the plains were covered by a hypothetical ocean at some point in Mars' history and putative shorelines have been suggested for its southern edges. Today these mildly sloping plains are marked by ridges, low hills, and sparse cratering. Vastitas Borealis is noticeably smoother than similar topographical areas in the south. In 2005 the European Space Agency's Mars Express spacecraft imaged a substantial quantity of water ice in a crater in the Vastitas Borealis region. The environmental conditions at the locality of this feature are suitable for water ice to remain stable. It was revealed after overlaying frozen carbon dioxide sublimated away at the commencement of the Northern Hemisphere Summer and is believed to be stable throughout the Martian year. A NASA probe named Phoenix landed safely in a region of Vastitas Borealis unofficially named Green Valley on 25 May 2008 (in the early Martian summer). Phoenix landed at 68.218830°N 234.250778°E. The probe, which will remain stationary, collected and analyzed soil samples in an effort to detect water and determine how hospitable the planet might once have been for life to grow. It remained active there until winter conditions became too harsh around five months later. Surface Unlike some the sites visited by the Viking and Pathfinder landers, nearly all the rocks near the Phoenix landing site on Vastitas Borealis are small. For about as far as the camera can see, the land is flat, but shaped into polygons. The polygons