Not all of them are dead. Centuries-old moss was able to spring back to life in the warmth of the laboratory. So too, incredibly, were tiny 42,000-year-old roundworms.

These fascinating glimpses of organisms from Earth’s long distant past are revealing the history of ancient ecosystems, including details of the environments in which they existed. But the melt has also created some concerns about ancient viruses coming back to haunt us.

“Melting will not only lead to the loss of those ancient, archived microbes and viruses, but also release them to the environments in the future,” researchers write in a new study, led by first author and microbiologist Zhi-Ping Zhong from Ohio State University.

Thanks to new metagenomics techniques and new methods for keeping their ice core samples sterilized, the researchers are working on getting a better understanding of what exactly lies within the cold.

In the new research, the team was able to identify an archive of dozens of unique 15,000-years-old viruses from the Guliya ice cap of the Tibetan Plateau, and gain insights into their functions.

“These glaciers were formed gradually, and along with dust and gases, many, many viruses were also deposited in that ice,” said Zhong. These microbes potentially represent those in the atmosphere at the time of their deposit, the team explains in their paper.

Past studies have shown microbial communities correlate with changes in dust and ion concentrations in the atmosphere, and can also indicate climate and environmental conditions at the time.

Within these frozen records of ancient times, 6.7 kilometres (22,000 feet) above sea level in China, the researchers discovered that 28 of the 33 viruses they identified had never been seen before.

“These are viruses that would have thrived in extreme environments,” said Ohio State University microbiologist Matthew Sullivan, with “signatures of genes that help them infect cells in cold environments – just surreal genetic signatures for how a virus is able to survive in extreme conditions.”

Comparing their genetic sequences to a database from known viruses, the team found the most abundant viruses in both ice core samples were bacteriophages that infect Methylobacterium – bacteria important to the methane cycle within ice.

They were most related to viruses found in Methylobacterium strains in plant and soil habitats – consistent with a previous report that the main source of dust deposited on Guliya ice cap likely originates from the soils.

“These frozen viruses likely originate from soil or plants and facilitate nutrient acquisition for their hosts,” the team concluded.

While the specter of ancient viruses seems particularly worrisome mid-pandemic, the greatest danger lies in what else the melting ice is releasing – massive reserves of sequestered methane and carbon. But it’s clear the ice could also hold insights into past environmental changes, and the evolution of viruses too.

“We know very little about viruses and microbes in these extreme environments, and what is actually there,” says Earth scientist Lonnie Thompson, who notes we still have many important questions unanswered.

“How do bacteria and viruses respond to climate change? What happens when we go from an ice age to a warm period like we’re in now?”

There is much still to be explored.