The biggest mass extinction the Earth ever saw was caused by catastrophic climate change triggered by volcanic eruptions, scientists say.
Temperatures rose 10degC after greenhouse gases were belched into the atmosphere by the Siberian Traps 252 million years ago, causing what's known as the Great Dying.
More than 95 percent of life was wiped out, paving the way for the dinosaurs.
New evidence volcanoes were behind it has been discovered by scientists, who uncovered a layer of ancient mercury in rock that dates to the period.
"Volcanic activities, including emissions of volcanic gases and combustion of organic matter, released abundant mercury to the surface of the Earth," said lead author Jun Shen of the China University of Geosciences.
"Typically, when you have large, explosive volcanic eruptions, a lot of mercury is released into the atmosphere," added Thomas Algeo, professor of geology at the University of Cincinatti.
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It's believed around 3 million cubic kilometres of ash was ejected into the air out of Siberia, about 100 million more times than was ejected during the infamous Eyjafjallajökull eruption of 2010.
The spike in mercury was found in 12 locations around the planet.
"We're often left scratching our heads about what exactly was most harmful. Creatures adapted to colder environments would have been out of luck," said Algeo. "My guess is temperature change would be the number one killer. Effects would exacerbated by acidification and other toxins in the environment," such as mercury.
"The longer this went on, the more pressure was placed on the environment."
The Earth has been through five mass extinctions, and many scientists believe we're on the verge of the sixth.
"What we should learn is this will be serious business that will harm human interests so we should work to minimise the damage," said Algeo. "It's much easier to address these problems before they reach a crisis."
A separate study last year using computer models also concluded catastrophic climate change was behind the Great Dying.
The latest findings were published in journal Nature Communications.