Back in 2013, scientists conducting geothermal drilling in the south Australian outback came across something exciting underground. Over a mile into Earth’s crust, they found traces of rock that had long ago been transformed into glass. It was evidence of extremely high temperatures and pressure—likely caused by a major impact endured in Earth’s distant past. It was heralded at the time as the third-largest impact zone ever found.
But now, a team of scientists has announced that the crater identified in 2013 is only part of the story. After investigating another scar in the mantle to the west of that initial discovery, they found that the same mass caused both craters. The discovery, recently published in the journal Tectonophysics, doubles the impact zone to over 250 miles wide, making it the biggest meteorite impact ever identified.
Geophysicists believe the meteorite broke in half just before crunching into Earth’s surface hundreds of millions of years ago, creating a kind of twin impact.
“The two asteroids must each have been over 10 kilometres [over 6 miles] across—it would have been curtains for many life species on the planet at the time,” lead researcher Andrew Glikson said in a news release.
But here’s where a mystery comes in—the team has not yet been able to identify a coordinating mass extinction event.
With the crater’s visible features long ago erased and judging from surrounding rock, they suspect the asteroid came plummeting at the planet around 300 million years ago or even earlier, but they can’t be sure. Other tremendous meteorite strikes—like the one that hit 66 million years ago and is often blamed for killing the dinosaurs—have left evidence of an ash plume via sediment in the world’s rocks. But so far, a rock layer revealing the effect of the collision discovered in Australia hasn’t been found.
Scientists will keep working to nail down the details of the impact: “Large impacts like these may have had a far more significant role in the Earth’s evolution than previously thought,” said Glikson.