15,000 years ago, some water froze atop the Tibetan Plateau and became part of a glacier. While humans were busy domesticating dogs, the ice entrapped millions of microscopic organisms per square inch. Many of the tiny life forms died, and their genomes—the only proof that they had been there in the first place—slowly degraded. Then, in 2015, scientists from the U.S. and China drilled down 50 meters into the glacier to see what they could find.

Five years later, these researchers have recovered evidence of ancient viruses in the glacier ice, including 28 viral groups that are new to science. Their study detailing the discovery was posted online as a pre-print on Tuesday.

Records of ancient microbes, like those found in glacier ice, give scientists a glimpse into Earth’s evolutionary and climate history. As our planet undergoes climate change, these frozen records can inform predictions about which microorganisms will survive, and what the resulting environment will look like.

“Glacier ice harbors diverse microbes, yet the associated viruses and their impacts on ice microbiomes have been unexplored,” the authors wrote in the paper. The group declined to comment on the paper, as it has not yet been peer-reviewed—“This is an exciting new area of research for us,” co-author Lonnie Thompson said in an email.

Viruses found in glacial samples known as ice cores are especially understudied because of how small they are, said Scott O. Rogers, a professor at Bowling Green State University and an author of the book Defrosting Ancient Microbes: Emerging Genomes in a Warmer World.

“The biomass is so low that anything you contaminate it with on the outside is going to be at much higher concentrations than anything on the inside of the ice core,” Rogers said. “The decontamination issues are extremely important; otherwise, you're going to just get garbage.”

According to the study, there are no special procedures used to avoid contamination when drilling, handling, or transporting ice cores. A key feature of the research was devising and testing a three-step process to remove these surface contaminants. In a -5ºC room, researchers used a bandsaw to scrape away 0.5 centimeters from the circumference of the cylindrical ice section. After, they washed the ice twice, first with ethanol and then water.

The researchers tested their protocol by covering the surfaces of sterile ice core sections with bacteria, viruses, and genetic material. In all cases, the procedure successfully removed the contaminants.

After applying the protocol to two ice cores from the Tibetan Plateau, the researchers used microbiology techniques to record the remaining genetic information lodged in the glacier ice. They found genetic information belonging to 33 different groups of viruses, 28 of which were completely new.

It is not surprising that dozens of these viruses had never been seen before, said Chantal Abergel, a researcher in environmental virology at the French National Centre for Scientific Research.

“We are very far from sampling the entire diversity of viruses on Earth,” she said.

But the effects of man-made climate change may make it impossible for scientists to discover many of the ancient viruses preserved in glacier ice. According to the study, warming temperatures are causing glaciers around the world to shrink and release microbes and viruses that have been trapped for tens to hundreds of thousands of years.

“At a minimum, this could lead to the loss of microbial and viral archives that could be diagnostic and informative of past Earth climate regimes; however, in a worst-case scenario, this ice melt could release pathogens into the environment,” the authors wrote.

The worst-case scenario seemed to become a reality in 2016, when an outbreak of anthrax in Siberia killed over 2,000 reindeer and hospitalized 96 people. Anthrax spores can stay alive for years, and the outbreak is believed to have been caused when melting permafrost thawed a decades-old deer carcass infected with the bacteria.

Frozen viruses may cause similar problems: Abergel and her husband led a team that revived a 30,000-year-old giant virus from permafrost, showing that it could still infect its target, a single-celled amoeba. She said that the reactivation of ancient viruses is a concern, but people should not become overly paranoid since viruses are “all over the place” and many pose a more serious risk to bacteria than humans.

Rogers had a more dire take. In a chapter of Defrosting Ancient Microbes, he and his co-author described the pathogens, hazards, and dangers associated with glacier ice research.