Hellas Planitia is the largest impact basin on Mars; it lies in the planet's southern highlands. Hellas Planitia – the roughly circular feature that dominates the image to the right – is about 2,300 kilometers (1,430 miles) across, or half the size of the United States. It was formed by a giant impact during the late heavy bombardment period of the solar system, some 3.9 billion years ago. Hellas Planitia also contains the lowest elevation point on Mars, reaching about 9 km below the surrounding highlands.

The Soviet probe Mars 2 crashed into Hellas Planitia at approximately 45°S 302°W in 1971, thus becoming the first human-made artefact to reach the martian surface.