sentences:  Stretching about across and covering an area of 470,000 km2, Olympia Undae is the largest continuous dune field on Mars.It is similar in size to the Rub' Al Khali in the Arabian Peninsula, the largest active erg on Earth.Olympia Undae lies within the informally named Borealis basin (also called the north polar basin), the largest of three topographic basins that occur in the northern lowlands of Mars.