sentence: Permafrost can be thousands of feet thick.
sentences_before: Tundras do not get very much rain or snow. However, they are very wet in the summer time because, three feet or so below the surface, the ground is always frozen. This permafrost is a layer of ice with the hardness of stone that keeps water from soaking into the earth beneath it.
sentences_after: In the summer the ground on top thaws out when the sun shines and the snow that did fall melts. There is no place for the water to go, except to form lakes and puddles on top of the ground until it freezes again or dries up.
concept_name: permafrost
headings: Arctic
categories: Arctic