How the earthworm digs its tunnel?

The Earthworms spend most of their lives digging tunnels in to soil. They can burrow their way in to even hard ground simply by using strength of the muscles. They contract and expand in a rhythmic manner to force an aperture in the ground and then they push on with their head. The earthworm swallows some of the soil he moves through. The earthworm then expels the digested soil and leaves it as a worm cast. It has been estimated that the yearly deposition above the ground of soil by earthworm is between 7 and 16 tons per acre in England.