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French scientists have announced the discovery of a giant 30,000-year-old virus, which was found buried 100 feet deep in the Siberian permafrost — a thick layer of soil that remains frozen year-round.

Mollivirus Sibericum — "soft virus from Siberia" — measures 0.6 micrometers, which means that, unlike normal virus specimens, it can be observed under a regular microscope. Viruses larger than 0.2 micrometers in diameter are considered "giant" viruses.

Several giant viruses have been discovered in the past decade, and although they have all turned out to be harmless, the findings have sparked fears that one day, humans could unearth a dangerous prehistoric pathogen.

"It's possible that viruses from similar viral families, or previously discovered virus families, turn out to be [harmful] pathogens," said Jean-Michel Claverie, head of France's Structural and Genomic Information laboratory (IGS), a unit of the National Center of Scientific Research (CNRS), in Marseille.

Claverie, one of the researchers who helped discover Mollivirus Sibericum, told VICE News that scientists had previously believed the permafrost to be a sterile environment.

"Today we know that's not the case," he said. Claverie and his team published their findings this week in the journal of the US National Academy of Sciences.

Under the permafrost lies a treasury of hydrocarbon reserves and precious minerals, and accelerated erosion as a result of climate change has made drilling easier.

"It's not that the permafrost itself is melting, but global warming is opening up [previously frozen] sea routes to sought-after sites that contain gas, gold, and minerals," said Claverie.

"In the event of mining operations, millions of tons of these layers will be dug up and exposed to air. All the conditions will be in place for the reactivation of these viruses, some of which could be pathogens."

The first giant virus ever discovered was the Mimivirus, which was mistakenly identified as bacteria by British microbiologist Timothy Robotham, because of its size. Claverie's team identified the micro-organism as a virus in 2003, in collaboration with the Research Unit in Infectious and Tropical Emergent Diseases, which is headed by top French researcher Didier Raoult.

In 2013, Claverie and his wife — researcher Chantal Abergel — discovered Pandoraviruses. One of the viruses from the family was discovered in seawater off the coast of Chile in 2011, and another in a freshwater pond in Melbourne, Australia.

The next year, Claverie and Abergel discovered the Pithovirus Sibericum virus in a 30,000-year-old specimen of Siberian permafrost. The discovery raised concern about the unearthing of previously undiscovered and potentially pathogenic viruses. Mollivirus — the latest virus to be thawed — was discovered in the same ice sample.

Mollivirus Sibericum particle towards the end of maturation (transmission electron microscopy) (Image via IGS, CNRS-AMU)

The viruses discovered so far in the permafrost are harmless to humans and animals, which is just as well, as Claverie's laboratory is not equipped to handle infectious diseases. However, the presence of both viruses in the same sample of ice proves that the permafrost can hold "many different kinds of viruses," according to the researcher.

In order to identify a virus, the laboratory introduces the samples to amoebas — single-celled animals, such as plancton. If the amoeabas die, scientists can deduce that they were exposed to a virus, which can then be isolated. The virus is then tested on mice and human cells to determine whether or not it is dangerous.

In order to be revived, a frozen virus needs only two things: for the ice to thaw, and contact with a susceptible host. And while the samples studied so far have revealed a low concentration of viruses, "All you need is one virus to ensure viral multiplication" said Claverie.

Follow Lucie Aubourg on Twitter @LucieAbrg