Scanning electron microscopy of two isolated Mollivirus particles. Credit:PNAS However, the authors of the study said that "the fact that two different viruses could be easily revived from prehistoric permafrost should be of concern in [the] context of global warming". In their paper, published in the Proceedings of the National Academy of Science , the researchers wrote: "Our finding suggests that prehistory 'live' viruses are not a rare occurrence." Further, they warn "we cannot rule out that distant viruses of ancient Siberian human populations could re-emerge as Arctic permafrost layers melt and/or are disrupted by industrial activities". One of the researchers, Jean-Michel Claverie, said: "Some viral particles which are still infectious can, in the presence of a receptive host, [lead to] the resurgence of potentially pathogenic viruses in Arctic regions". He said that climate change is making the Arctic region more accessible for the extraction of mineral and oil resources, which could further expose such viruses. "If we are not careful, and we industrialise these areas without putting safeguards in place, we run the risk of one day waking up viruses such as smallpox that we thought were eradicated," Professor Claverie told Agence-France Presse.

Mollivirus particle at a late assembly stage. Credit:PNAS M. sibericum is the fourth giant virus discovered. The first of the giant viruses, Mimivirus, was found in 2003. The second family of giant viruses, the Pandoravirus, was found in 2013 in a pond in suburban Melbourne, at La Trobe University in Bundoora. Edward Holmes, a professor of infectious diseases and biosecurity at the University of Sydney, was cautious of the findings. Microbiologist Professor Ben Cocks in 2013 at the La Trobe University pond where the Pandoravirus was found. Credit:Ken Irwin "What's interesting [about the new virus] is viral diversity; it's cool that they're so big, and I'm querying the age, but the risk? None."

"We are more at risk from the standard microbiological fauna that floats around," he told Fairfax Media. Representatives of the four 'giant' virus families: Mollivirus, Megavirus, Pithovirus and Pandoravirus under a scanning electron microscope. Credit:PNAS Professor Holmes said: "The problem with things this old is that DNA degrades quite quickly. So trying to get any material that is that old is very, very difficult. "The oldest pathogen [we have identified] is plague bacterium going back 1500 years - and that was was very degraded. Here you have an intact virus going back 30,000 years so it would have to have been absolutely, instantaneously frozen and [then experience] no thawing or degradation. That's a tough call for me ... I'd like the age to be independently verified." Professor Edward Holmes, University of Sydney.

However, Professor Holmes said he isn't saying they're wrong: "If it is 30,000 years old, it's fantastic." Viruses have traditionally been thought of as small and simple organisms. However, the discovery of the Mimivirus in 2003 means that we now know viruses can be large and complex. Viruses are considered "giant" if they are longer than half a micron, or 0.0005 millimetres. Scientists hope that analysis of these giant viruses will help us better understand viral evolution and perhaps the origins of life itself. Professor Holmes said that humans first studied viruses that threatened humans and economically important animals and plants. However, it is now understood that we have just "scratched the surface of the virus-sphere". "There is an amazing diversity of viruses out there. They're in weird organisms, they're of different shapes and sizes - the giant viruses came as a real surprise - and the sea is full of them. And in most cases they have nothing to do with disease whatsoever," Professor Holmes said.

"We've just missed an amazing diversity of life. Most DNA on earth is certainly in viruses." The discovery of a prehistoric giant virus will no doubt trigger memories for some of sci-fi thriller, The Thing, in which scientists operating in the Antarctic disturb a long-dormant, violent alien species. Of course, M. sibericum is non-sentient, Arctic and terrestrial, so perhaps we probably don't have too much to worry about.