The early Earth was hit by a fast moving asteroid so large that its impact likely made the oceans boil.
It would have kicked off tsunamis thousands of metres high and gouged out a crater big enough to swallow the island of Ireland whole.
The impact that convulsed the planet and wiped out the dinosaurs 65 million years ago would have been a minor event in comparison.
It involved an asteroid about 10 km across, but the massive rock that ploughed into the Earth 3.26 billion years ago was estimated by US scientists to have been between 37 and 58 kilometres wide.
"We knew it was big, but we didn't know how big," stated Prof Donald Lowe of Stanford University, a co-author of a study to be published in Geochemistry, Geophysics, Geosystems , a journal of the American Geophysical Union.
It would have been moving at a fearsome clip, perhaps 20 kilometres per second.
It would have triggered earthquakes that echoed back and forth across the planet's surface for at least half an hour, the scientists say.
The Earth is estimated to have formed 4.54 billion years ago so it was quite young when this early impact occurred.
It would likely have been one of many impacts however.
It occurred during the later years of the Late Heavy Bombardment period three to four billion years ago when the inner solar system was peppered with stray asteroids looking for a place to land.
Most of the evidence for the mammoth collision disappeared as the planet’s surface renewed itself through movement of the Earth’s crust and ordinary erosion over time.
This means that the scientists do not have a ground zero to go study.
“We can’t go to the impact sites. In order to better understand how big it was and its effect we need studies like this,” said Prof Lowe.
The scientists had to make do with what geological evidence they could find to understand what happened to the Earth during this time, he said.
This was found in rock formations in the Barberton greenstone in South Africa and in western Australia that hinted at an asteroid impact.
By studying these sites he and colleagues were able to estimate the size of the asteroid needed to result in these rock formations and then model the results of such an impact.
The effects at the surface would have been catastrophic, the scientists said. The sky would have gone red hot with vaporised rock that was blown around the globe.
This would have liquefied and then become solid to rain down on the surface. The shockwaves would have left their mark on the Barberton greenstone, some of the oldest rocks on the planet.
The microscopic life that was struggling at the time to take hold on the surface would mostly have been wiped out. But clearly some survived to feed into the evolutionary processes that followed to deliver complex life despite the impact.
