28 prehistoric viruses unknown to science were found in a glacial ice cores. They are thought to be around 15,000 years old.

• Scientists revived viruses trapped in the ice caps in the Tibetan mountains 15,000 years ago.
• Twenty-eight of these ancient viruses were unknown to science.
• They are unlikely to be dangerous to humans, and probably affected plants.

Scientists have been able to revive 33 types of ancient viruses, which are thought to have been frozen in the ice of the Guliya ice cap in the Tibetan Plateau 15,000 years ago.

Of those, 28 were unknown to science.

The viruses, which were found in an ice core collected in 2015, were able to survive in the ice for thousands of years.

However, these viruses are unlikely to be harmful to humans, as their genetic makeup suggests they would have more likely originated from soil or plants, not animals or humans.

The findings were published in the peer-reviewed journal Microbiome last month.

Older viruses, and more complex organisms, have been revived from the ice before

Although reviving ancient viruses sounds like the start of an apocalypse movie, there's little need to worry. The process has been carried out before with no negative consequences.

In 2014, scientists found a virus thought to be 30,000 years old inside a tiny animal called a protozoan, frozen in Siberian permafrost.

The virus is related to, but distinct from, a type of virus found today.

In a study published in 2015, another virus, named Mollivirus sibericum, was identified and revived from the same sample.

More complex, ancient organisms have also been revived from ice.

Earlier this year, scientists published a study in which they thawed a tiny worm-like animal called a Bdelloid rotifer, captured in Siberian ice 24,000 years ago. The rotifer, pictured below, was immediately able to eat and reproduce.

Microbes freed from thawing ice could be harmful, but more recent ones are a bigger cause for worry

The scientists who identified 28 new viruses in the ice said the findings could teach us about the effect of climate change on viruses.

"We know very little about viruses and microbes in these extreme environments," said Lonnie Thompson, an author on the study and scientist at the Byrd Polar and Climate Research Center.

"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?" he said.

As climate change warms the glaciers and ice caps around the world, more microbes captured in the ice, like viruses and bacteria, are at risk of being released.

But microbes that caused modern recent epidemics prompt the most concern.

One chilling example is an outbreak in Russia that happened after a heatwave in 2016.

A 12-year-old boy died and at least twenty people were hospitalized from catching anthrax.

Officials at the time said that they believe the bacteria had been released from a frozen reindeer carcass that was trapped under Siberian permafrost since the last outbreak, which was 75 years before, in 1941.

"Permafrost is a very good preserver of microbes and viruses because it is cold, there is no oxygen, and it is dark," evolutionary biologist Jean-Michel Claverie at Aix-Marseille University in France, told the BBC in 2017.

"Pathogenic viruses that can infect humans or animals might be preserved in old permafrost layers, including some that have caused global epidemics in the past," he said.