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enormous impact basin in the southern hemisphere of Mars and the planet's largest recognizable impact feature. Centred at roughly 40 S, 290 W, Hellas measures about 7,000 km (4,400 miles) across, including the broad elevated ring surrounding the depression, and 8 km (5 miles) deep. Its floor, covered with partly eroded sediments, is the lowest place on Mars. The basin was probably created by collision with an asteroid very early in Mars's history, not long after the planet formed.