During the Carboniferous period the Earth was very warm and moist, covered with dense forests. When the Carboniferous rainforests collapsed, most of the trees were immediately covered with mud, as everything was wet and swampy. The trees are one of the biggest natural storage for carbon. Because the trees and the large amounts of carbon trapped in them were covered, lot of carbon was removed from the atmosphere, thus from the carbon cycle. Gradually, the carbon from the trees ended up deep into the ground, which eventually turned into coal. The removal of such a high quantity of carbon from the carbon cycle resulted in a reduced Greenhouse effect, which in turn made the planet much colder.