In the far north permafrost effectively seals the ground preventing surface water from soaking in and draining away. Recently however, warming in the arctic (Siberia in this example) has reduced the permafrost enough to allow water to escape from some of the small lakes that cover the landscape. The right side of this image was captured in 1973, the left in 2002. The large lake bisected by the yellow line has shrunk considerably in 30 years. For more information about these Landsat photos see the Earth Observatory site. A comparison of the source images (links below) shows many other lakes, large and small that have shrunk.