The most prominent dark marking (albedo feature) on Mars, roughly triangular, about 1000 km wide and 1200 km long, centred at +10° lat., 290° W long. It is the Greek name for the Gulf of Sirte on the north African coast, whose shape it resembles, and means ‘the great sand-bank’. It is an area of dark basalt rock sloping gently downwards from west to east with a variable covering of wind-blown dust. It contains regions with hydrated minerals, evidence for surface water in the past. It was first sighted by C. Huygens in 1659. The official name of this surface region is now Syrtis Major Planum.

http://photojournal.jpl.nasa.gov/catalog/PIA00173