hit pay dirt: achieve good fortune or success; find what you are looking for
In the 1850s, pay dirt was real dirt—the nuggets of gold left in the pan when miners panning the streams had washed out all the soil and other debris. These were the forty-niners, adventurers who rushed to California when gold was discovered in John Sutter's mill. They spent their days sifting through the gravel of the American River, where the mill was located, hoping to strike pay dirt. A few were even successful.
By the 1880s, pay dirt was being used as a metaphor for money. A line from the November 1884 Century Magazine says "He lives . . . in a style that proves that he has lots of pay dirt somewhere." Since the twentieth century, hitting pay dirt can mean achieving any kind of desired result, such as winning a game, getting a raise, or finding something you've been looking for. The scoring areas of sports fields, such as the football end zone, are also sometimes referred to as pay dirt.