In 1938, the Philadelphia Athletics third-base coach Russell "Lena" Blackburne waded into a tidal tributary in the Delaware River and realized he was soaking in a solution for one of baseball's biggest problems.
Back in the 1930s, baseball was a much more dangerous sport than it is today. Newly made balls were slick, and pitchers had a hard time controlling their tosses to home plate. Teams tried to improve each ball's grip by scuffing the hide with bleacher dirt, tobacco juice, shoe polish, or even licorice. This was less than ideal. Umpires complained that these applications made the ball easier to tamper with—indeed, these alterations are illegal today because they can alter the physics of a ball's movement—and players moaned that the applications were inconsistent.
Those inconsistencies had consequences. Over the course of a game, scuffed-up balls often got much dirtier and softer—making them not only more difficult to control, but also more difficult to see. With the invention of batting helmets still decades away, ballplayers were taking a risk with their life each time they stepped into the batter's box. In fact, a Cleveland Indians shortstop named Ray Chapman was killed in 1920 after he was beaned in the head by an errant pitch.
So when Coach Blackburne came across a slick patch of mud near his hometown swimming hole, his mind went straight to the playing field. The goop was gritty, but it resembled a mixture of "chocolate pudding and whipped cold cream." He toted some of the gunk back home and found that, sure enough, it smudged the ball perfectly, enhancing the grip without damaging the leather. When Blackburne showed the result to American League umpires, they gave the application a thumbs-up. By the 1950s, every major league team was using it.
Now before every major and minor league game (as well as many college games), an umpire or clubhouse attendant wipes a light coat of Blackburne's magic mud on each ball used. In fact, it's a rule in the major leagues. According to MLB Rule 3.01, all regulation baseballs much be "properly rubbed so that the gloss is removed" [PDF].
The mud even has fans outside of baseball. According to The Washington Post, "half of NFL teams buy Lena Blackburne mud to help their players grip the ball."
Though it's rumored to be located somewhere on the banks of the Delaware River near Palmyra, New Jersey, the mud hole's exact location remains a closely guarded secret. Only one person, Jim Bintliff, the mud's solitary farmer, knows exactly where to find it—and he refuses to give clues as to its location. "Does Jim Bintliff wave a magic wand over the mud during the winter, or add some mysterious ingredients to it?" the mud's website asks. "That too is a dark secret. He'll never tell."