North Polar Basin (Mars)
The Borealis Basin, or the North Polar Basin, is a large depression or basin in the northern hemisphere of Mars.  The North Polar Basin takes up 40% of the Martian surface. It has a diameter of about 9,500 km (5,900 miles), but is slightly stretched, or elliptical, meaning its true size is closer to 10,600 km (6,600 mi) by 8,500 km (5,300 mi).