Some dams built by beavers are up to two kilometres and exist probably from one million years.  What are these dams used for?

The beavers co-habit with several beaver families in lodges whose entrance is found underwater.  To maintain the level of water and consequently protect their nest, the beavers construct lodges.  For this purpose, they make use of trunks of trees, branches and aquatic plants, sometimes even small rocks which they are able to carry between their heads and front legs simultaneously moving forward using their hind legs.  The lodge is then reinforced by a layer of mud.

An adult beaver can cut down in only half-an-hour a twelve centimetre wide tree.  It uses its sharp incisors like scissors.

Image via Wikipedia