Two French researchers have discovered "giant viruses" they believe have been buried in the Siberian permafrost for as long as 30,000 years.

Jean-Michel Claverie and Chantal Abergel of Aix-Marseille University in Marseille, France, led the team that found a previously unknown virus called Pithovirus sibericum that is still viable, National Geographic reported.

Writing about the study in the Proceedings of the National Academy of Sciences, Claverie and Abergel detailed the unusual specimens, which are 1.5 microns in length and 0.5 microns in diameter. The Pithovirus sibericum can be seen under a regular microscope and has surpassed the 1-micron-long virus previously discovered by Claverie and Abergel.

Their size and toughness have helped the viruses stay alive for thousands of years even buried deep in the permafrost. The researchers noted that as climate change shifts the environment, any kind of ancient virus could come to the surface--perhaps even one that would infect people.

"Among known viruses, the giant viruses tend to be very tough, almost impossible to break open," said Claverie and Abergel. "Special environments such as deep ocean sediments and permafrost are very good preservers of microbes [and viruses] because they are cold, anoxic [lacking oxygen], and in the dark."

The researchers tested the specimens by exposing amoebas in their lab to the giant viruses. They discovered that the virus could still infect the host cell quickly.

"We use amoeba on purpose as a safe bait for capturing viruses. We then immediately verify that they are not able to infect animal/human cells," they said.

While some have raised concerns that a virus could be unearthed that infects humans, a microbiology professor says the risk is actually quite small.

"The idea would make a great movie but is extremely unlikely unless the virus came from a frozen human being who possibly died from a virus that is no longer in circulation," Edward Mocarski, a professor of microbiology at Emory University, told National Geographic by email.

"A very small proportion [of the viruses on Earth] represent viruses that can infect mammals and an even smaller proportion pose any risk to humans."

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