An ancient virus has come back to life after 30,000 years lying dormant in the Siberian permafrost.
The pathogen is the largest of a type of giant virus discovered 10 years ago which can be seen under a microscope, unlike most viruses.
But while the Pithovirus sibericum pathogen does not infect human or animal cells, instead attacking single-cell amoebas, scientists warn it could be a harbinger of more ancient viruses being unleashed as frost melts.
Scientists are now working to establish if there are any more viruses thought to have been extinct in Siberia's permafrost, which is retreating and reducing in thickness, that might threaten humans.
Dr Chantal Abergel of the Centre of Scientific Research at the University of Aix-Marseille in France (CNRS), said, "We are addressing this issue by sequencing the DNA that is present in those layers. This would be the best way to work out what is dangerous in there." But the experts warn that digging down into the permafrost, which is being eyed for potential mining for natural resources, could be a huge mistake as it may expose new viral threats.
Professor Jean-Michel Claverie, also of the CNRS, said, "It's a recipe for disaster. If you start having industrial explorations, people will start to move around the deep permafrost layers. Through mining and drilling, those old layers will be penetrated and this is where the danger is coming from."
Speaking to the BBC he warned the ancient forms of the smallpox virus, which was officially deemed eradicated 30 years ago, could resurface.
He said, "If it is true that these viruses survive in the same way those amoeba viruses survive, then smallpox is not eradicated from the planet - only the surface."