The researchers warned it may just be the tip of the iceberg:

'One-quarter of the Northern Hemisphere is underlain by permanently frozen ground, referred to as permafrost,' researchers wrote in the paper.
'Due to climate warming, irreversibly thawing permafrost is releasing organic matter frozen for up to a million years, most of which decomposes into carbon dioxide and methane, further enhancing the greenhouse effect. Part of this organic matter also consists of revived cellular microbes (prokaryotes, unicellular eukaryotes) as well as viruses that remained dormant since prehistorical times.'