A virus frozen in the Siberian permafrost for the past 30,000 years has come back to life.
Pithovirus sibericum was discovered by French scientists when a deep layer of frost thawed.
It is not dangerous to humans or animals, but its revival raises the possibility of other more deadly viruses such as smallpox being exposed amid global warming.
"It has important implications for public health risks," said France's National Centre for Scientific Research (CNRS).
"The revival of viruses that are considered to have been eradicated, such as the smallpox virus, whose replication process is similar to that of Pithovirus, is no longer limited to science fiction.
"The risk that this scenario could happen in real life has to be viewed realistically."
The virus was found buried 30 metres (100ft) down in frost.
It is part of a family of giant viruses discovered 10 years ago that are so big they can be seen under a microscope.
At 1.5 millionths of a metre, Pithovirus sibericum is the biggest virus ever discovered, but it has not infected anything since mammoths and Neanderthals walked the Earth 30,000 years ago.
The virus infects amoebas, but does not attack human or animal cells.
But Professor Jean-Michel Claverie, from CNRS, said the exposure of permafrost to global warming and industrial exploration was now a "recipe for disaster".
Researchers are examining DNA from the frost to see what else might lurk within.
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