Researchers have revived a giant virus more than 30,000 years old, recovered from the permafrost of north-east Siberia.
The virus poses no threat to people, although it is considered a giant when compared to other viruses as it is microscopic and infects amoebas.
The one from Siberia is a new kind of giant virus, joining a group that was first discovered 10 years ago.
The researchers said their finding suggests that dangerous germs might emerge in the future as permafrost thaws because of global warming or mineral exploration.
They said sampling permafrost to look for ancient viruses that infect amoebas is an inexpensive and safe way to assess that potential threat.
The finding was reported in the Proceedings of the National Academy of Sciences.