Hellas is an enormous crater, formed four billion years ago by an asteroidal impact. The giant patera, called Hellas Planitia, had a diameter of proximately 2100 kilometers, being 7-12 times greater than the Chixculub Crater.
The energy released by the collision must have been gargantuan, outstaging the 500 zetajoules of the impact that exterminated the dinosaurs in the Cretaceous-Tertiary extinction, sweeping away the atmosphere on impact. It'd even been hypothesized that one or both of Mars' two moons had formed upon this impact, in much the same way Earth's moon formed when, 3,8 billion years ago, Theia, a planet the size of Mars, collided onto the Earth, and the ejecta of the impact eventually coalesced to form the moon. Because the asteroid which caused the Hellas Basin was much smaller, however, the result would obviously be less spectacular. Whether Phobos or Deimos were created upon an asteroidal impact or captured into Mars orbit from the nearby asteroid belt, however, was only known when the composition of the moons' and Mars' interior were compared.
It was so immense that the shock waves it sent through the planet gave rise to the antipodal Tharsis Bulge, which in turn created a rift in the planet's surface now known as Valles Marineris. Because the asteroid was so massive, however, it was less influenced by Mars' gravity, so that it didn't crash straight onto it. As the slightly ovoid shape of Hellas Planitia indicates, it hit the planet from the east-southeast, which explains why the Tharsis Bulge isn't diametrically opposed to it.
This phenomenon had already occurred twice on Earth, almost wiping out all life for good: it's no coincidence that the Deccan traps, a volcanic plain in India and China, lie exactly on the other side of the world relative to the Chixculub Crater. The Siberian Traps, originally having been as extensive as the Amazon Forest with a surface of 7 million km², likewise are antipodal to the Wilkes Land crater in West-Antarctica, responsible for the Permian-Triassic extinctions. The Siberian Traps, just like the Wilkes Land crater with a diameter of roughly 500 km, are much larger, and indeed the Permian-Triassic extinctions of 245 million years ago were of far greater gravity, wiping out most life on Earth for good: 70% of all terrestrial species and 96% of all marine species were extirpated. Reportedly, this extinction killed some 99,5% of all individual organisms, almost finishing the biosphere. We've been lucky the asteroid wasn't as massive as that which gave rise to the Hellas basin.
Indeed, Mars may have born life even before the terraformation hadn't the asteroidal impact made the planet uninhabitable to even the most primitive life. Asteroids are dangerous: of all extinction event marking the end of geological periods, eight have been proposed to have had a geological cause (Cambrian-Ordovician, Ordovician-Silurian, Silurian-Devonian, Devonian-Carboniferous, Carboniferous-Permian, Permian-Triassic, Triassic-Jurassic, Tertiary-Quaternary), while four out of nine have been proposed to have had an astronomical cause (Silurian-Devonian, Devonian-Carboniferous, Permian-Triassic, Triassic-Jurassic, Cretaceous-Tertiary). Asteroidal impacts have been responsible for the two most extensive and most famous exterminations of the history of the Earth, and had nearly accounted for all life in the solar system. Had the asteroid which struck the Earth in the Permian-Triassic extinction been a bit more massive, our magnificent civilization likely would never have arisen.
But once civilization was born, society developed far too quickly for it to be exterminated by an event of geological or astronomical timescale: within seven thousand years from the dawn of the Sumer civilization around 6000 before transhumanism, nothing in nature could still harm us. In seven millennia, we'd become invincible, immortal to the wrath of nature. Hurricanes, deluges, volcanoes, earthquakes, meteors, comets, asteroids, stars, or even the cosmos - nothing could still harm us.
Indeed, we could even outlive the universe, for we have a hundred billion years until all stellar activity ceases. At the rate technology advances, we could even forestall the universe's death. And even if technology would continue its snail pace, 100.000.000.000 times this year's progress would turn us into a God.
