On Saturday night, the greatest Olympic swimmer of all time made his London Games debut — and bombed. In the 400-meter individual medley race, Michael Phelps came in an embarrassing fourth place, while American rival Ryan Lochte took the gold. This is the same Phelps who set a world record in that same race four years ago in Beijing, where he won a record eight gold medals, giving him a total of 14 golds and two bronze medals. With the silver he won in Sunday's 4x100-meter men's relay, Phelps is two medals shy of being the most decorated Olympian ever. But after his humiliating loss Saturday, says Mark Sappenfield at The Christian Science Monitor, there's a new nagging "question no one was asking before the games began: What happened to Michael Phelps?" Here, five theories: 

1. Phelps is out of shape
After his record-shattering run in Beijing, Phelps was burned out — and it showed on Saturday, says Bill Plaschke at the Los Angeles Times. He "admittedly has never loved the physical stress of this event," considered the marathon of swim contests, but the magnitude of his loss is mostly a testament to his lack of fitness, mental and physical. He did little training for the two and a half years after the last Olympics, and even teammates have questioned his recent lack of exercise. Sad to say, Phelps' "greatness may have ended on those training mornings he didn't want to get out of bed."

2. He didn't want the gold bad enough
Phelps has been swimming for Olympic gold since the Sydney games in 2000, so "maybe he's starting to slow just a bit from the wear and tear" of 12 years of brutal competition, says Tim Dahlberg of The Associated Press. Or "maybe he didn't have the fire inside for training." It's hard to escape the sense that he "seemed almost giddy" at the idea that Friday marked his last practice as a competitive swimmer. "Phelps was always taking a chance that he might be swimming in one Olympics too many," and maybe he should have quit while he was ahead — and excited about winning.

3. Phelps is just warming up
Yes, Phelps "found himself in a malaise after Beijing," but "Lochte revived his drive," says Linda Robertson at The Miami Herald. When the deceptively laidback Lochte started beating him consistently, Phelps started training again in earnest, with the goal of making his final splash in London. It's taken longer to get in top form than he expected, but "given how much Phelps loathes losing, Lochte and the rest of the competition better brace themselves." With a handful of races left to swim, the much-heralded rivalry between the two Americans "will make more waves before it's over."

4. His emotions are getting in the way
Before the Olympics even began, "Phelps said he was going to have to manage his emotions in this, his last meet," says The Christian Science Monitor's Sappenfield. Competitive swimming has been his life, and at 27, it's a big deal to move on to something new. So it's a good bet that "he's letting those emotions get the better of him."

5. Phelps is only human
Whatever was going on in Phelps' head on Saturday, we all learned that "the guy with the most gold medals in history is just a man, not a merman," say Josh Levin and Justin Peters at Slate. Maybe he's on the decline, or maybe it's just that the high-end, hydrodynamic swimsuit he set all his records in four years ago has since been banned from use. But paradoxically, being human "might actually increase his marketability." And if he can win a gold or two in the coming days, it sets up "a hell of a comeback narrative."