For tens of thousands of years the Siberian permafrost has locked away relics of the past, from wooly mammoth bones to seeds. As climate change contributes to rising global temperatures, scientists are digging up more and more finds that used to be unreachable through the frozen earth – including several kinds of giant viruses.

Most common viruses are incredibly tiny and typically only have about four genes or so, not even enough to be considered “alive” in the same way as a bacterium or a human cell. Giant viruses, on the other hand, are a different story. Sometimes as big as bacteria, giant viruses have much more genetic material, anywhere from hundreds to thousands of genes, Michael Byrne writes for Vice Motherboard. And as the permafrost melts, scientists are finding them more and more frequently.

According to a new report, a group of microbiologists from several French and Russian institutions have just discovered a second new species of giant virus in a single sample of Siberian permafrost in just two years. Both, according to lead researchers Jean-Michel Claverie and Chantal Abergel, are viable even after 30,000 years of being frozen in the ground.

"A few viral particles that are still infectious may be enough, in the presence of a vulnerable host, to revive potentially pathogenic viruses," Claverie tells AFP. "If we are not careful, and we industrialise these areas without putting safeguards in place, we run the risk of one day waking up viruses such as small pox that we thought were eradicated."

Claverie and Abergel were intrigued when they heard that a group of Russian scientists had managed to revive a seed found buried in the permafrost since the Late Pleistocene era. Wondering what else they might find, the husband-and-wife team began to probe the Russian sample, using amoebas as bait for any viruses that might have survived the deep freeze, Jen Christensen writes for CNN.

“Every once in a while, we see them die and that's when we know somebody must be killing them," Claverie tells Christensen. "This way, we know which to isolate from the others."

The fact that they are still viable after such an immense period of time raises concerns for Claverie and Abergel that melting permafrost and Siberian mining operations could unleash more harmful pathogens, Byrne writes. As Claverie and Abergel note in the study:

"Although no read sequences were close enough to detect known Poxvirus and Herpesvirus isolates in the metagenome of our permafrost sample...we cannot rule out that distant viruses of ancient Siberian human (or animal) populations could reemerge as arctic permafrost layers melt and/or are disrupted by industrial activities.”

While most discovered giant viruses exclusively target amoebas, at least one strain has been known to infect humans. But despite the drama of these discoveries, other researchers say that people should be less worried about the possibility of finding pathogens in the permafrost.

“This is a theoretical possibility, but in the absence of any evidence that this might be dangerous, I think we should worry about the viruses that could jump out of mammals on Earth right now,” Vincent Racaniello, a virologist at Columbia University tells Chris Mooney for The Washington Post.

At the end of the day, there is no immediate danger from these ancient viruses, although it might be worth keeping an eye on anything else that comes out of the permafrost. In the meantime, these relics could give scientists new insights into what life was like tens of thousands of years ago.