The ice caps are melting, and they may hold some unpleasant surprises.

Over the last 100 years, global temperatures have risen, and the ice caps—floating packs of ice, mostly found around the North and South Poles— have started to melt. According to NASA, the planet has been shedding about 13,500 miles of ice per year since 1979. To put that in perspective, that’s an area larger than the state of Maryland.

The numbers are alarming for several reasons, but one unexpected threat may soon take precedence: According to the BBC, melting permafrost soils are exposing “ancient viruses and bacteria,” and in some cases, those microorganisms are springing back to life.

This isn’t just a theory. During a heatwave in northern Russia, an anthrax outbreak seemed to spring out of nowhere, infecting at least 90 people and killing 2,300 reindeer. After studying the outbreak, scientists theorized that the deadly disease returned to the Siberian tundra when permafrost melted, infecting the water and soil in the area.

It was certainly a strange situation, but scientists worry that it might not be an isolated case. Anthrax was actually common in reindeer populations up until the later part of the 20th century; many ancient reindeer remains are likely infected with the disease, and under the right conditions, warm temperatures could help anthrax spring back to life.

That’s only part of the problem.

Scientists are also worried about other diseases that might be lurking in the arctic regions of the Earth. Granted, viruses and bacteria would likely be associated with biological matter, so Antarctic ice caps probably wouldn’t hold disease—but Antarctic ice caps are actually growing, according to NASA estimates. It’s the ice in other parts of the world that’s melting.

In 2015, scientists found a 30,000-year-old “giant” virus in the Siberian permafrost, then warned that it was still infectious. While the virus wasn’t thought to be a threat to humans (and it could actually hold some important clues as to how life appeared on Earth), it’s another example of how microorganisms can spring back into existence after thousands of years in the ground.

In some cases, scientists bring back ancient viruses on purpose.

In 2005, NASA restored microbes frozen in the Pleistocene period. Scientists have also restored microbes thought to be more than 100,000 years old, so time doesn’t seem to be much of a factor for ancient viruses and bacteria.

There’s also the case of the smallpox victims buried in Siberia; in the 1890s, a major epidemic ravaged the country, and many of the remains were buried in the northern territories. Melting ice could bring the virus back, creating an immediate public health crisis.

Those are just some of the viruses and bacteria we know about, but there could be other, more mysterious microbes hibernating. Some might be resistant to current antibiotics and antiviral medications. Others will be completely harmless, of course.

But that highlights the big question: What else is out there, waiting in the permafrost?