After killing his family in an insane rage, Herakles (also known as Hercules) received orders from the oracle at Delphi to perform 12 labors to purge his sin. These were assigned by Eurystheus, king of Mycenae.
The fifth labor was clearly impossible. Augeas, king of Elis, was rich in horses, cattle, goats, anything that could eat, and... The products of 3000 oxen, gathered lovingly for ten years, stood there, caked hard, higher than a house. The smell was so bad, no-one wanted to go near. Herakles had to clean this up. Fighting monsters never looked so good.
Herakles spoke to Augeas and agreed to clean his stables in exchange for a tenth of the cattle. Herakles sent the river Alpheus through the stables, clearing them in less than a day.
Augeas refused to pay, claiming Herakles had not performed the job with his own hands. When his own son took the side of Herakles, he banished him. Herakles made war on Augeas, deposing him, and replacing him with his son.
Click here for the sixth labor.