Phrases from Mythology: Sisyphean Task

In ancient Greek stories, Sisyphus was an evil king whose punishment after death was to roll a very large stone to the top of a steep hill. Each time he got near to the top of the hill, the stone rolled down to the bottom, and he had to start again, and he had to continue doing this for ever. A very difficult job that seems impossible to finish is sometimes described as a "Sisyphean task".