Planum Australe is the southern polar plain on Mars. It extends southward of roughly 75°S and is centered at 83.9°S 160.0°E / -83.9; 160.0. The geology of this region was to be explored by the failed NASA mission Mars Polar Lander, which lost contact on entry into the Martian atmosphere. (wikipedia entry)
A planum is a plateau or high plain. On Mars they are named after the nearest named albedo feature on the Schiaparelli or Antoniadi maps.