In some places on Earth, most of the ground remains frozen - even in summer. The upper 2 meters (6.5 feet) might thaw under the warmth of the sun, but will frost back up as soon as autumn arrives.

Ground that is at or below the freezing point for two or more consecutive years is called permafrost. Such land is estimated to cover about 20 percent of the Earth's surface.

Most of the world's permafrost is located in Arctic or Antarctic regions, including Siberia, northern Canada, Alaska and Greenland. It's also found at high altitudes, like in the Andes or even on Africa's highest peak, Mount Kilimanjaro.

But global warming has been increasing permafrost temperatures, thus affecting its thickness. According to the IPCC, the thickness of the permafrost in grounds in southern Alaska has decreased 4 millimeters per year since 1992.

But what are the consequences of melting permafrost?

Zombie bacteria

After the death of a young boy and more than 20 other illnesses from anthrax in Siberia this past week, it has been speculated that a recent heat wave has awoken anthrax bacteria that had been lying dormant within the carcasses of long-deceased reindeer buried in the permafrost.

Anthrax bacteria can survive in the form of spores for decades

"The anthrax spores lay in wait in the permafrost for more than a century," an agriculture watchdog was quoted in the newswires as saying.

And indeed, anthrax bacteria do form spores in asexual reproduction. These spores are highly resilient, and can survive temperature extremes over decades and even centuries. They come back to life when conditions become amenable for them - like in thawing permafrost.

Other bacteria form spores as well. Many of those bacteria are also pathogens, or produce deadly toxins - like the tetanus bacterium or Clostridium botulinum. Fungi are also known to be able to survive in permafrost for a very long time.

A study found 1 to 1,000 million bacteria per gram of soil from Spitsbergen in Norway. A quarter of the total number of bacterial cells were still viable after defrosting.

Reawakening viruses

In 2014, French and Russian researchers found and reactivated a gigantic - and still unknown - virus from Siberian permafrost.

This giant virus dozed for 30,000 years in permafrost

With a diameter of 1 micron, the virus is a thousand times bigger than all other viruses known to scientists today. But it's completely harmless to humans.

The authors of the study believe it was frozen in permafrost for more than 30,000 years. But once defrosted, it was able to infect an amoeba, its natural host.

As viruses are not alive, they cannot die either.

Heat can destroy viruses. Low temperatures, however, don't harm viruses at all. "Viruses like it cold, dark and wet," Thomas Mertens, virologist at Ulm university hospital, told DW.

The authors of that study warned that "thawing of permafrost either from global warming or industrial exploitation of circumpolar regions" could cause "future threats to human or animal health."

Permafrost is a pretty good preservative agent, indeed.

Russian researchers were even able to revive an Ice Age flower from frozen ground - and it was still fertile.

Wildfires in permafrost regions present a possible feedback loop

Greenhouse gas feedback loop

Permafrost is made of dead plants and animals that have been frozen for thousands of years. Estimates are that, worldwide, permafrost contains half of all organic material in soils.

When permafrost thaws, that organic material is exposed to warmer temperatures. This makes it decay, releasing large amounts of methane and carbon dioxide into the atmosphere. Many fear that melting permafrost could create a feedback loop, further accelerating global warming.

Wildfires also possibly contribute to accelerating thaw of the permafrost, scientists found.

Here again, a dangerous feedback loop could come into play: Defrosted regions are more likely to see increased wildfires in the future.