The Borealis Basin, also known as the North Polar Basin is a large depression or basin in the northern hemisphere of Mars. It covers 40% of the planet. Some scientists think the basin formed during the impact of a single, large body (~2% the mass of Mars) with a diameter of about 1,900 km (1,200 miles). However, the basin is not currently called an impact basin by the IAU. The basin is one of the flattest areas in the Solar System. The landing site of the Viking 1 lander is a bay that opens into this basin.