It started with a bang – a big bang – when a massive space rock slammed into shallow waters off Mexico 66 million years ago.
The 14-kilometre-wide asteroid punched a massive hole in Earth's crust and sent a cloud of ash and dust into the atmosphere.
As the planet plunged into darkness and acid rain fell in the months and years that followed, about three-quarters of plants and animals – including most dinosaurs – were snuffed out.
Now scientists have revealed more about what happened in the first few minutes and days of this cataclysmic event, known as the Chicxulub impact.
The asteroid flung huge amounts of the ocean floor into space and triggered the mother of all tsunamis.
The first global simulation of the mega-tsunami, published today in the journal AGU Advances, models how it would have ricocheted around the globe.
"The first 10 minutes were just crazy," said co-author Brian Arbic, an oceanographer at the University of Michigan.
"Within seconds, stuff is just blown miles high and the Earth is depressed by many miles and the whole ocean is thrown up into the air."
The first hours and days
To model the first 10 minutes of the Chicxulub event, the team relied on techniques normally reserved for the study of impacts on places like the Moon and Mercury.
Planetary scientist Brandon Johnson of Purdue University found that after the big splash, the material that fell back to Earth caused waves several kilometres high.
"I was surprised to see just how powerful it was from the point of impact," Dr Johnson said.
Then, the team used tsunami-propagation models to track the giant waves across the ocean and around the world.
While the Gulf of Mexico copped the worst of it, most of the world's coastlines felt the brunt within two days.
Some parts of the world were shielded from the tsunami, which sped across the deep ocean like a jetliner, but others like Australia and New Zealand were not.
And despite waves slowing down as they approached the shore, they were still very powerful.
"The Chicxulub tsunami wave heights were so much greater than those in the 2004 [Boxing Day] tsunami, which meant that the energy was about 30,000 times greater," Professor Arbic said.
A tsunami expert on the team, Vasily Titov of the National Oceanic and Atmospheric Administration, said the simulation suggested the impact could have been "pretty brutal" for Australia.
It showed waves of about 20 metres would have hit the east coast within 24 hours after the tsunami was triggered, Dr Titov said.
And while the exact level of inundation of coastlines has yet to be modelled, he estimated areas up to 40 metres above sea level would have been deluged.
This is the maximum inundation seen right at the source of the 2004 Boxing Day tsunami, he added, and is twice the inundation level experienced in Japan by the 2011 tsunami.
With no tsunami warning system, a few dinosaurs would have no doubt been swept away by the wave, long before most species were wiped out by the full planetary impact of the asteroid strike.
Geology backs up computer simulation
The researchers back up their model with physical evidence that came from the analysis of 120 samples of ancient sediment, mainly collected from scientific ocean-drilling projects around the world.
"This tsunami was strong enough to disturb and erode sediments in ocean basins halfway around the globe, leaving either a gap in the sedimentary records or a jumble of older sediments," lead author Molly Range, a paleoceanography masters student at the University of Michigan, said in a statement.
"[Such disturbances] are consistent with our model results, which gives us more confidence in the model predictions."
The team found ancient sediments in areas where the tsunami was predicted to be travelling rapidly were more eroded than in areas where the waves moved more slowly.
New Zealand is one place where the tsunami appears to have had a significant impact.
Christopher Hollis, study co-author and geologist from Victoria University, said there was evidence in New Zealand of a "chaotic mass" of rock and mud that appeared on top of smooth ocean sediments from the time.
Until now, these were believed to have been caused by a submarine mudslide or some other local event.
Dr Hollis said the evidence from the model now suggested they were caused by "backwash" from the mega-tsunami triggered 12,000 kilometres away.
"The tsunami is thought to have ripped up chunks of sea floor."
'Many uncertainties remain'
Kliti Grice, of Curtin University is a geochemist, who has also studied the immediate effects of the Chicxulub impact, welcomed the research.
"[This is a] very timely paper and will create a lot of discussion among the research community," Professor Grice said.
But as far as she was aware, there has been no physical evidence of the Chicxulub impact tsunami found in Australia.
"Many uncertainties remain, and there is much room for improvement in future studies," she said.
James Goff, an expert in palaeo-tsunamis not involved in the study, agreed it was important to find physical evidence to back up models.
"If the model and existing physical data match, then you can use the model to guide you on where to look for more evidence, and so the iterative process begins," said Professor Goff of the University of New South Wales.
For now, though, he was impressed with the new paper giving a "real sense of just how big and how devastating" the tsunami was.
"It is a bit like the Arnold Schwarzenegger of the tsunami world," Professor Goff said.
"So to model the tsunami and track its impact globally is a marvellous piece of work."
