Hellas Planitia is a plain located within the huge, roughly circular impact basin Hellas located in the southern hemisphere of the planet Mars. Hellas is the third or fourth largest impact crater and the largest visible impact crater known in the Solar System. The basin floor is about 7,152 m east to west. It is centered at 42.4°S 70.5°E / -42.4; 70.5. (wikipedia entry)
A planitia is a low plain. On Mars they are named after the nearest named albedo feature on the Schiaparelli or Antoniadi maps.