Hellas Planitia, also known as the Hellas Impact Basin, is a roughly circular impact crater located in the southern hemisphere of Mars. Due to its size and contrasting light color, Hellas Planitia was one of the first Martian features discovered from Earth by telescope. With a diameter of about 1400 miles (2300 Km) - half the size of the United States -, it is the second largest confirmed impact structure on the planet. The basin is thought to have been formed during the Late Heavy Bombardment period of the Solar System, over 3.9 billion years ago, when a large asteroid impacted Mars.