Bombarded by meteors billions of years ago, the Martian surface
is covered in craters, especially in the southern hemisphere. Craters
aren't seen in abundance in the north, because it appears that huge
lava flows over that hemispere smoothed over cratered terrain after
the time when meteors rained down on the planet.The largest
crater on Mars is Hellas Basin, which would cover nearly half of
the United States!