In cold countries, the soil freezes hard in winter. In summer, the top few centimeters may thaw, while lower down the stays frozen. This permanently frozen soil is called permafrost. Several extinct woolly mammoths, which died about 45,000 years ago, have been dug out of the permafrost in Siberia. The meat of one of them, preserved in this natural refrigerator, was so fresh that dogs tucked into it!