Bioturbation

That's the process of soil (or other substances) being mixed up by the activity of animals. Worms are among the most important actors in this process, which is vital to the health of the ecosystem. Darwin actually studied this process, and his last book was The Formation of Vegetable Mould through the Action of Worms, with Observations on their Habits. On his front lawn he had a stone that he carefully observed daily, noting how the action of worms underground led to settling of the soil. Kaptain Kobold recreated the worm stone in celebration of Darwin Day earlier this year.