Viruses are responsible for some of mankind’s deadliest diseases, from smallpox and AIDS to influenza and dengue fever. Unfortunately, getting rid of them can be tricky. This week, scientists announced the discovery of a new virus—the world’s largest—that had survived for over 30,000 years trapped in Siberian permafrost. Though this particular virus poses no threat to humans, scientists say less benign specimens could be lying in wait.

Viruses are tiny, infectious agents with both living and nonliving characteristics. They cannot multiply on their own, instead doing so in the cells of other organisms—from animals to plants to bacteria. No one knew anything about them until 1892, when Russian scientist Dmitri Ivanovsky authored a paper on some of their defining traits. More than a century later, scientists came across the first so-called giant viruses, which, unlike their smaller relatives, can be observed with a regular microscope. These original giant viruses, known as Megaviruses, contain up to 1,200 genes, compared to about 10 genes in common viruses such as AIDS. A second family of giant viruses with even more genes, the amphora-shaped Pandoraviruses, was then documented last year in water collected off the coast of Chile.

The latest discovery, a third family of giant viruses called Pithoviruses, came to light Monday in a paper published in the Proceedings of the National Academy of Sciences. This Pithovirus is 1.5 micrometers long and 0.5 micrometers in diameter, the largest virus ever discovered, and infects single-celled amoeba. Exposed human and animal cells, the scientists learned through extensive tests, remain virus free. It has a similar shape but apparently behaves completely differently from Pandoraviruses. Even more notably, it was found nearly 100 feet underground in Siberian permafrost that had been radiocarbon dated to over 30,000 years ago, around the time Neanderthals disappeared.

Husband-and-wife researchers Jean-Michel Claverie and Chantal Abergel, the lead authors of the paper, said they came up with the idea of looking for viruses in Siberian permafrost after other scientists grew a plant from seed tissue found there. “If they could revive a plant, I thought there was no reason we couldn’t do the same thing with viruses,” Claverie explained in a phone interview from France. In order to perform their experiment, they received a shipment of permafrost that had been collected in 2000. Having now identified three families of giant viruses, Claverie said they appeared to be “opening up a window on a totally new world.”

Claverie speculated that viruses might remain viable in a frozen state for up to a few million years, before the Earth’s natural radioactivity damaged their DNA. To test that hypothesis, he and his team will attempt to revive amoeba-harming viruses from even further down in the permafrost. They are also searching for the genetic signature of viruses harmful to humans, though those wouldn’t be revived. “If the Pithovirus is capable of resisting 30,000 years of freezing, there is no reason to think that viruses that affect humans wouldn’t also do the same thing,” Claverie said.

As permafrost and sea ice rapidly melt due to global warming, more drilling and mining is expected to take place in the Arctic. And if smallpox, believed to have been eradicated in the 1970s, and other dangerous viruses are still out there, then this could be a “good recipe for disaster,” according to the researchers. “The fact that we might catch a viral infection from a long-extinct Neanderthal individual is a good demonstration that the notion that a virus could be ‘eradicated’ from the planet is plain wrong,” the researchers said. “At least a stock of vaccine should be kept, just in case.”