The cause of the biggest mass die-off seen on Earth -- couldn't even be seen with a naked eye.

That's because -- far from the gargantuan asteroids, volcanoes and coal fires previously implicated by researchers -- gas-belching microbes have come into focus as the real reason 90 percent of all life was wiped out hundreds of millions of years ago, say researchers from the Massachusetts Institute of Technology, otherwise known as MIT.

Fossil remains indicate that sometime around 252 million years ago, about 90 percent of all species suddenly disappeared, but it required a careful examination of the carbon levels in the atmosphere and environment back then for researchers to finally realize microbes were clearly the most likely culprits.

The microbial suspects are methane-producing archaea called Methanosarcina, which bloomed almost explosively in the oceans, spewing vast amounts of methane into the atmosphere and, as a result, dramatically altering the climate and chemistry of the oceans, according to findings published in the Proceedings of the National Academy of Science by researchers from both MIT and China.

Volcanoes apparently spurred the rapid growth of the microbes by ejecting an ample supply of the element nickel, an important nutrient source for the little critters.

The research team arrived at the microbial link after considering three major clues: First, geochemical evidence shows an exponential (or even faster) increase of carbon dioxide in the oceans at the time of the mass extinction. Second, genetic evidence shows Methanosarcina at the same time began spewing out enormous amounts of methane accumulated from carbon dioxide in the water. Finally, third, sediments show a sudden increase in the amount of nickel also deposited the same period.

While carbon deposits indicate a boost in the amount of carbon-containing gases -- carbon dioxide or methane -- produced at the time of the extinction, the MIT teams showed volcanic eruptions alone could not have introduced enough carbon into the atmosphere to account for the high carbon levels in sediments left from that era. As well, the carbon levels don't jibe with the volcanic activity back then.

"A rapid initial injection of carbon dioxide from a volcano would be followed by a gradual decrease," said MIT postdoctoral researcher Gregory Fournier. "Instead, we see the opposite: a rapid, continuing increase ... That suggests a microbial expansion."

The growth of microbial populations, he added, is among the few phenomena capable of increasing carbon production exponentially, if not even quicker.

The study explains the burst of methane would have increased carbon dioxide levels in the oceans, resulting in ocean acidification -- similar to the acidification predicted from the ongoing climate changes seen today.

Separate evidence suggests marine organisms with heavily calcified shells in particular were obliterated during the extinction period, which can be easily explained by water acidification.

MIT geophysics professor Daniel Rothman said, "A lot of this rests on the carbon isotope analysis," which is extraordinarily strong during the extinction portion of the geological record. "If it wasn't such an unusual signal, it would be harder to eliminate other possibilities."

While nothing yet has been found that can prove without a doubt the microbes, or anything else for that matter, caused the die-off, "the cumulative impact of all these things is much more powerful than any one individually," said Rothman.