The land in the Antarctic and near the Arctic is still covered by a continuous ice sheet thousands of metres thick. This mass of ice preserves the climatic records of the last hundreds of thousands of years of the Ice Age, showing how the ice sheet expanded and retreated over time while its thickness has changed too. The rate of glaciations and thaws was not uniform either. While the first half of the ice age was milder, in the second half the ice sheet reached the 54th parallel north.