Mars’s largest visible impact basin is Hellas Planitia, a vast depression in the planet’s southern hemisphere.
The basin formed when a large asteroid or comet struck Mars early in its history. Its floor is surrounded by a broad, roughly circular rim and lies several kilometres below the surrounding terrain, making Hellas one of the deepest lowlands on Mars.
Hellas Planitia is often called the Hellas impact basin because the broader geological structure includes the basin rim and surrounding ejecta. The name “Hellas” comes from the Greek name for Greece, while “Planitia” is a planetary surface term for a low plain.
Hellas is sometimes confused with Utopia Planitia, another enormous impact basin in Mars’s northern hemisphere. Spacecraft have studied Hellas to investigate Martian geology, atmospheric pressure variations, dust activity, and evidence of ancient water-related processes.