There have been many ice ages in the entire history of the Earth. During these cold phases, the average temperature of the Earth dropped 5 degree below the normal level and the polar regions were completely covered with ice. The last big cold period, known as 'ice age', ended around 10,000 years ago, with the thawing of the large glaciers that were several kilometers thick. It is not yet clear what causes an ice age. Researchers believe that the sudden change int he Earth's climate may be a result of some sort of a disturbance in the orbit of the Earth of a reduction in the heat of the sun reaching the Earth. Another theory suggests that, prior to the ice age, the continents might have changed positions and blocked or reduced the flow of warm currents from the equator to the poles, and thus allowed ice sheets to form in the region.