Late Heavy Bombardment
With a diameter of about , it is the largest unambiguous impact structure on the planet; the obscured Utopia Planitia is slightly larger. (The Borealis Basin, if it proves to be an impact crater, is considerably larger.) Hellas Planitia is thought to have been formed during the Late Heavy Bombardment period of the Solar System, approximately 4.1 to 3.8 billion years ago, when a protoplanet or large asteroid hit the surface.