About one-fifth of Earth’s land is permafrost, or ground that is permanently frozen. This classification is based entirely on temperature and disregards the composition of the land. It can include bedrock, sod, ice, sand, gravel, or any other type of material in which the temperature has been below freezing for over two years. Nearly all permafrost is thousands of years old. Permafrost is found in Canada, Russia, northern China, most of Greenland and Alaska, and Antarctica.