Ever since the Chicago Cubs installed an organ at Wrigley Field in 1941, music has been a part of the baseball ballpark experience. Unlike basketball, hockey or football, baseball doesn’t have non-stop action, and its frequent periods of inaction give an organist or stadium DJ a broad canvas on which to improvise. Initially, ballpark music was strictly organ music, as organs were present at every ballpark by the middle of the 20th century. Starting in the 1960s and 1970s, the organist was complemented by the stadium DJ, who played recorded music over the stadium sound system. Gradually, the DJ overtook the organist and many stadiums no longer play live organ music; a little less than half of the 30 ballparks that host Major League Baseball games today don’t employ an organist.

As technology improved, allowing stadium DJs to play music on the computer rather than on the record or 8-track player, teams began allowing baseball players to choose the music they wanted to hear. And, in the 1990s, the phrase “walk-up music” entered the sports fan’s vernacular. Position players chose songs for the stadium DJ to play when they walked from the on-deck circle to the batter’s box. Pitchers picked music to be played while they warmed up at the start of their outing.

Players select their music in a variety of ways. Some pick a song from their favorite artists, artists that often originate from their corner of the world (e.g. a lefthanded pitcher from Texas warming up to country music or Southern hip-hop; a utility infielder from the Dominican Republic opting for a merengue or bachata tune). Others will choose music they feel helps motivate them to perform better on the field, choosing a new song whenever they enter a slump. Some players will even allow the stadium DJ to pick their music. In 2007, New York Mets third baseman David Wright gave the fans the opportunity to vote for his walk-up song online, giving them list of 50 of his favorites to choose from (the fans choice: “Me & U,” the only hit by Cassie, a hip-hop artist).

While almost every baseball player has walk-up music, only a handful of players have had their music become an iconic part of their image and legacy. The best example of a player who’s become closely identified with his entrance music is New York Yankees pitcher Mariano Rivera.

Rivera has been the Yankees’ closer for more than a decade and a half, called upon to get the last three outs in the ninth inning when the Yankees are ahead by a handful of runs. The closer’s role is one of the most pressure-packed in all of team sports. Do it well, and your opponents often feel defeated as soon as you come into the game. Do it poorly, and you’ll frustrate your teammates, your manager and your team’s fan base. After all, nothing’s more demoralizing than losing a game you’ve all but won, and a closer’s job is to turn almost-certain wins into definite victories. Rivera is arguably the best closer in baseball history, an integral part of a Yankees team that’s won five championships and advanced to the playoffs in all but one of his 16 seasons.

During Rivera’s rise to prominence in the late 1990s, the DJ at Yankee Stadium, his home ballpark, began playing “Enter Sandman” by Metallica when Rivera came into the game. To a certain extent, “Enter Sandman” seems like a strange choice for Rivera’s entrance. While it does start off softly before building to a eardrum-thumping crescendo of guitars, “Enter Sandman” is a loud, heavy-metal song. Rivera, the most reserved of the Yankees superstars developed over the last two decades, is serene and soft-spoken. To the untrained ear, “Enter Sandman” sounds arrogant and boastful, two words that have never been used to describe Rivera.

However, in many ways, “Enter Sandman” does compliment Rivera. Its lyrics, about a child battling nightmares as he tries to sleep, are a perfect metaphor for Rivera, whose job is to gives opposing hitters nightmares while trying to put the game to bed in the Yankees’ favor. Rivera, a deeply religious man by all accounts, probably approves of the song’s encouragement to “Say your prayers little one” and its recitation of a classic children’s bedtime prayer (“Now I lay me down to sleep/Pray the Lord my soul to keep…”). In addition, Rivera may also get a kick out of the idea of opposing hitters saying their prayers before facing the dancing and darting fastballs that he throws with ruthless efficiency and pinpoint accuracy.

It also helps that Yankee Stadium is one of the loudest outdoor venues anywhere. Its combination of rabid fans and their New York-style obnoxiousness and hubris combined with an ear-splitting sound system that could wake the dead already makes the Bronx, NY stadium one of the toughest venues for visiting teams to compete. Combine those elements with a song that’s loud, even before you raise the volume, being played while an indomitable closer jogs to the pitcher’s mound from the bullpen behind the outfield fence and makes his warm-up throws and it isn’t hard to understand why many enemy teams are mentally vanquished before they even take their cuts against Rivera, whose entrance routine serves to both whip the Yankee Stadium crowd into a frenzy and intimidate opponents.