French scientists have awakened a giant stone age virus - the world's biggest - locked deep under the ice of Siberia for at least 30000 years.
The virus, Pithovirus sibericum, which poses no danger to humans or animals, is so large it can be seen under a conventional microscope and dates back to the times when mammoths and Neanderthals walked the earth.
"This is the first time we've seen a virus that's still infectious after this length of time," said Professor Jean-Michel Claverie, from the National Centre of Scientific Research, who revived the virus with Chantal Abergel, his fellow researcher and wife.
He added that 60% of its gene content did not resemble anything on earth.
Though the virus is harmless, the scientists warned that others might be lurking under the permafrost and be exposed by drilling or global warming.
Modern viruses are tiny and have few genes. The flu virus, for example, has 13 genes and is about 100 nanometres across. Pithovirus sibericum has 500 genes.