In the long, storied history of Fourth-Place Medal's Investigative Unit (founded: Monday), one question has been asked by our readers more than any other. Today, on our five-day anniversary, we will attempt to tackle the biggest Olympic mystery of the Beijing Games: what is Michael Phelps listening to on his iPod?

In nearly every camera shot of Michael Phelps on dry land, he can be seen with iPod headphones dangling from his ears. The earbuds are a ubiquitous presence in the ready room and on the starting block; they're just as much a part of Phelps' 'uniform' as goggles and a swim cap. About two minutes prior to the start of a race, Phelps sheds the iPod along with his warm-ups. So, what is he listening to?

Podcasts of NPR's This American Life and Dylan live at The Supper Club. No wait, that's my iPod. Phelps listens to hip-hop music on his. He says it helps motivate him before a race.

While his pre-race tracklist varies, Phelps has said that "I'm Me" by Lil' Wayne has been on his playlist in Beijing. The track, off Weezy's mega-hit "Tha Carter III" features the line:

Yes I am the best/and no I ain't positive I'm definite/I know the game like I'm reffing it

Other artists that populate Phelps' iPod include: Jay-Z, Young Jeezy, Eminem and Outkast. (What, no 'Pac?) Occasionally, he'll throw some techno into the mix, but usually keeps things rap-centric. Phelps doesn't speak much about the specific songs he's listening to, but he did tell NBC in 2004 that Eminem's "'Til I Collapse" was on his most-played list at Athens. In 2005, he created a playlist for the website Rhapsody that included the songs "Roses" by Outkast, "Burn" by Usher, "Overnight Celebrity" by Twista and  "Smile" by G-Unit.

Mystery: solved.

The Investigative Unit takes weekends off to watch that 'Saved by the Bell' where they go to the murder mystery theater and Lisa is on it. But we'll be back Monday, so if you see a mystery that needs solving, leave a note in the comments section and we'll get to it soon.

Photo via Getty Images