Acidalia Planitia (Mars)

From classical albedo feature at 44N, 21W; "Sea of Acidalia", named for the fountain Acidalia where the Graces bathed

Acidalia Planitia is a plain on Mars. Most of this region is found in the Mare Acidalium quadrangle, but a small part is in the Ismenius Lacus quadrangle. The plain contains the famous Cydonia region at the contact with the heavily cratered highland terrain. (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.