Threats will come from all sides. Some will grab him, strangle him while he swims. Some will wait until the lights have dimmed and the fans have left. Every threat will eat at his energy and strength, until he comes up for air after his final race and realizes that his threats—or his dreams—are gone.

Swimming might never be wildly popular in America. But for those who consider sports a test of human limits, there may be no more impressive feat than what Michael Phelps will do over nine days in August. He will swim eight finals (five individual, three relays) and 17 races overall, including prelims and semis, in a quest to become the only Olympic athlete to win eight gold medals at a single Games. Football, basketball, hockey and baseball players rest not only after games, but during games. Tennis and soccer players get days off between matches. Boxers get months. But Phelps? Phelps will burn a marathon's worth of calories in the pool every day for nine days, on his way to swimming more than 30 miles. He will weaken with every minute, stroke and breath. The threats will not.

THREAT 1: PAIN
Phelps begins with the most difficult event: swimming's decathlon, the 400 individual medley. The race begins with 100 meters of butterfly, in which he must propel his body out of the pool, over and over, until he feels as if he's doing squat jumps with two kids on his back. The fly requires an edge, almost an anger. "You have to be tougher, meaner," says 1992 gold medalist Mel Stewart. "If you don't have a base of strength and stamina, you fade. You die."

Next, the backstroke. Lie on your back, put ankle weights on and kick for a full minute. That's what the backstroke feels like. By the end of these 100 meters, a swimmer's quads and abs are shot. The race is half over.

Switch to breaststroke, Phelps' weakest. He will struggle to hold form: back straight, elbows tight, head up, wrists snapping just so. His arms will whine and the field will close in and someone might even pass him, as Ryan Lochte did in the trials.

The freestyle leg will take anything Phelps has left. During breaststroke, muscles lock up. Still, he must sprint for 50 more seconds. Many swimmers drive the final 25 meters without lifting their head to breathe, to wring the final tenths out of the clock. At trials, Matt Grevers saw spots and felt his consciousness start to slip away. Phelps broke the world record to barely win the event at trials, and he called it "one of the most painful races of my life." He has 15 left.

PHELPS' ANSWER:
He feels pain, but Phelps, 23, has a not-so-secret antidote: the world's best dolphin kick, which rockets him through the water on the butterfly leg at three meters per second—faster than any other swimmer. So why was the 400 IM at trials so difficult? According to his coach Bob Bowman, Phelps didn't taper completely, so he didn't have the full physical reserve and muscle strength he'll have in Beijing. In other words, Phelps broke the world record without a full tank of gas.

THREAT 2: FATIGUE
Between warmups, racing and warmdowns most mornings at the Games, Phelps will swim roughly 4,000 meters—the equivalent of running 11 miles—by noon. He needs a nap, but time is consumed by medal ceremonies, drug testing and media demands before he finally boards a shuttle to athlete housing. That's right: Phelps stays in the Olympic Village, not a cushy hotel. And he'll take a bus instead of a limo. He won't have easy access to his family, either. In Athens, he once called his mom to complain he was alone while the other swimmers were having fun. Then, as now, he wasn't allowed out without a security detail.

Phelps hopes to get to bed by 10, although he was awake much later in Athens. Then he must sleep, which means forgetting what he's accomplished that day and the expectations of tomorrow. "You worry about the pressure," says Katie Hoff, who also won five events at trials. "Thinking about it makes me nervous, and too much of that can get to you." Every night, rest becomes more and more difficult to achieve, as pain builds and teammates who have finished can exhale and distract him. Yet Phelps must awake fresh. Unlike at every other major meet, finals in Beijing will be held in the morning so they can be aired live in the U.S. in prime time.

PHELPS' ANSWER:
He has done this before—in Athens, where he swam eight events and won six golds and two bronzes; at the 2007 worlds, where he won seven golds; and at trials, where he won all five individual events. Since Athens, Bowman has fine-tuned the schedule so Phelps wastes little time and no effort between events. But mostly it comes down to Phelps: His ability to focus and refusal to acknowledge exhaustion are legendary. "He starts doing everything really precise," says teammate Erik Vendt, "and that just builds up until the event."

Michael Muller

His flexible ankles (he can make a straight line from knees to toes) and size 14 feet give Phelps flipper-like propulsion.

PHELPS' ANSWER:
In Athens, Bowman didn't sleep the night before the 100 butterfly, worried the frenzy had caught up with his star. It seems the media onslaught, the wait for the shuttle and a random drug test forced him off his schedule. But Phelps won anyway, even after trailing with 25 meters left. Others become distracted when their routines are thrown off. Concentration blown, they race opponents or fear or expectations. Phelps' greatest gift is his ability to compartmentalize, to focus on what he's doing and race only himself.

THREAT 4: MIND GAMES
As the Games go on, mental demons will take their toll. "I swam four times in eight days in 1984," says NBC commentator Rowdy Gaines. "I was a basket case." On the way to the pool before an event in 1972, Mark Spitz wondered whether he was about to swim the 100 fly or the 200 free. Even Phelps battled mood swings in Athens, once bickering with Bowman about how quickly he needed to board the shuttle to the pool. Later, after he won gold, Phelps was so overwhelmed that Bowman had to drag him to the warmdown pool. No matter how experienced Phelps may be, he still can be overwhelmed by the mental stimulation of the Olympics.

PHELPS' ANSWER:
Routine. Because he will be so busy in Beijing, Phelps will have little time for reflection, which will help him ride out emotional upheavals. "Once I get to the competition, it's like…" Phelps pauses, " …it's like I'm in a cage, so just let me out, because I know exactly what to do. I know how to prepare myself. I know how to warm up. I know how to get my head in the game. Competition is my favorite part of the sport. That's what I do best."

THREAT 5: DOUBT
Baseball players don't have to worry about Babe Ruth and Lou Gehrig looking over their shoulders, but many of swimming's legends still roam pool decks. Matt Biondi showed up at trials and pointed out how different Beijing will be from Omaha. He even threw down a prediction: "There's going to be an upset in Beijing. The human body has limits." The great Spitz, who won a record seven golds in Munich, has been very supportive. But even he wonders aloud how the constant competition from Lochte and Ian Crocker will wear on Phelps. The swimming world won't bet against Phelps, but many wonder if winning eight golds is actually possible.

PHELPS' ANSWER:
He loves doubt. Craves it. Phelps scours newspapers for challenges he can cut out and paste on his wall. Before Athens, he hung a poster of Ian Crocker above his bed so he could stare into his rival's eyes every night. When retired Aussie legend Ian Thorpe said he didn't think anyone could win eight races at a single Olympics, Phelps memorized the quote to motivate him for Beijing. His target times for these Games are scribbled on a piece of paper that he won't show even his mom. "I want to do something that's never been done," he says. "That's what's on that paper."

Chinafotopress/Getty Images

Beijing's Water Cube will host Phelps' nine-day, 30-mile, eight-event swimming odyssey.

PHELPS' ANSWER:
It used to be common for swimmers to retire in their early 20s, and Phelps has said in the past that he'll retire by 30. But training methods and nutrition have changed, and let's not forget the motivation Phelps gets from the $5 million or so he makes annually as a swimmer. Phelps' desire to do the impossible won't subside next year, or anytime soon. Bet on this: He will compete in London at age 27, and again in Chicago, if the city wins the bid, for the 2016 Games. Phelps' legacy will not be written until long after Beijing.

THREAT 8: THE UNEXPECTED
Challenges to Phelps' quest could come from any direction. He could catch a cold. His shuttle bus could break down. Water could flood his beloved LZR Racer and sink him. Or the challenge could be nothing. At trials, Brendan Hansen, the Olympic bronze medalist and American record-holder, failed to qualify for Beijing in the 200 breaststroke and said this: "I don't know. I came in and had a really good feeling in warmup and everything." Don't think Phelps isn't aware of the possibility that he could have an off day or two.

Getty Images

Phelps has set 22 individual world records, starting with his first in 2001.

He will likely leave China with more career gold medals than any Olympian in history (track star Carl Lewis is one of four athletes with nine). To the media, he'll be a topic of debate about the greatest ever. To kids, he'll be an idol or a heartthrob. To Speedo, a cash cow. To fellow swimmers, a source of awe. But with his seared muscles and dazed mind, with his screaming joints and heavy head, Michael Phelps will be exactly what the ancients intended back when it all began. He will be an Olympian, in the truest sense of the word.