Scientists have revived a “zombie” virus they say spent nearly 50,000 years frozen in permafrost, part of a new batch of research that identifies several newly discovered viruses that have been frozen underground for tens of thousands of years.

To better understand the risks posed by frozen viruses, Jean-Michel Claverie, a French professor of medicine and genomics, tested earth samples taken from Siberian permafrost – also called permanently frozen ground – in search of what he describes as “zombie viruses.”

Permafrost can be found on land and below the ocean floor in areas where temperatures rarely rise above freezing. It’s often found in the Arctic, Greenland, Alaska, Russia, China and Eastern Europe.