Similar to montane grasslands, polar Arctic tundra can have grasses, but high soil moisture means that few tundras are grass-dominated today. However, during the Pleistocene glacial periods (commonly referred to as ice ages), a grassland known as steppe-tundra or mammoth steppe occupied large areas of the Northern Hemisphere. These areas were very cold and arid and featured sub-surface permafrost (hence tundra) but were nevertheless productive grassland ecosystems supporting a wide variety of fauna. As the temperature increased and the climate became wetter at the beginning of the Holocene much of the mammoth steppe transitioned to forest, while the drier parts in central Eurasia remained as a grassland, becoming the modern Eurasian steppe.