Scientists say there could be something under frozen ground that contains many deadly diseases. Ancient microbes have been trapped for millennia in the permafrost that covers large areas of Earth. Permafrost is ground that stays completely frozen for at least two years. It is common high in mountains and near the North and South Poles. The scientists say the permafrost is melting because of climate change. The result could be the release of unknown, harmful viruses, which scientists are calling 'Factor X'. Birgitta Evengard, a professor in infectious diseases at a university in Sweden, said we know little about Factor X and the biological dangers trapped in Earth's permafrost.

Virologist Jean-Michel Claverie says there are viruses deep in the permafrost that are more than 50,000 years old. He said these could be harmful to humans. Professor Claverie said "ancient viruses up to a million years old" would be the most terrifying because they are unknown and we have never encountered them before. He added that deadly bacterial diseases like smallpox and anthrax could be released on a large scale. Factor X could pose even bigger problems. However, many scientists agree that a bigger danger from melting permafrost is the release of methane. This is a greenhouse gas that is 80 times more harmful than CO2.