Hercules and the Augean Stables
Hercules had to clean out the horse stables of King Augeus, which hadn't been cleaned in years and were all full of horse poop. It seemed like it would take forever, but Hercules found a river nearby and dammed it up so it ran right through the stables and washed them all clean. This picture of it, from a metope of the Temple of Zeus at Olympia (about 460 BC) shows Hercules digging, while Athena shows him what to do.
