When I visited Colorado Springs in mid-May, Bowman was wrapping up a three-week high-altitude training stint with Club Wolverine, the elite Ann Arbor, Mich., swimming organization at which he bears the title High Performance Coach and whose members include Phelps. (From 2004 until his resignation earlier this year, Bowman was also the head coach of the University of Michigan’s swim team; Phelps followed Bowman to Ann Arbor from his native Baltimore.) Training in thin air is a proven and perfectly legal means of boosting an athlete’s red-blood-cell count, which increases the oxygen delivered to muscles. The trip also afforded Bowman, who prefers to start his day at around 4 a.m., the opportunity to lead his crew through a final pre-Olympic-trials boot camp. The schedule for the Colorado sojourn featured three sessions in the pool per day and an additional hour of “dry land” activities like weight training or Pilates, for a total of 70 workouts in three weeks. Although Bowman was dedicated to mixing up the training regimen to keep his swimmers from getting comfortable, he followed certain patterns: the early session featured 90 minutes of low-key, continuous aerobic exertion  three or four miles of wake-up laps. Midday practice was an intense two-hour affair, putting the swimmers through their paces at top speeds or at the very threshold of their endurance; dry-land work followed for an hour. Later in the afternoon, the day’s final workout focused on muscle power rather than lung power, featuring drills with parachutes, fins, paddles, kick boards, floats, limb-disabling bands, snorkels and other accouterments designed to isolate particular skills. Bowman could be sure that his swimmers had little time or energy left at the end of the day for anything but eating, sleeping and occasionally slumping in front of the television. Phelps, who is said to require 8,000 to 10,000 calories a day to sustain his efforts, spent much of his free time napping or in pursuit, as he puts it, of “whatever I want to eat, whenever I want it, however much I want.”

The practice I attended was what Bowman termed a farewell swim: the two dozen team members would be packing their bags afterward and heading to Santa Clara, Calif., before dawn the next morning for a meet. Consequently, Bowman had designed a feel-good wind-down session, featuring lap after lap at what he called a low-intensity “white pace,” at which swimmers maintain heart rates of around 130 beats per minute. (Trained swimmers have heart rates that are much lower both at rest and at maximum exertion than those of ordinary fit people.) The swimmers seemed to drift by in slow motion, plying their backstroke with languid windmill motions, then moving on to freestyle, bending their elbows and dropping their forearms through the water’s surface, hatchet-style, then rising and falling from the pool with their breaststroke and butterfly. Music played over a loudspeaker, but the sound bounced off the water and the walls so haphazardly that it registered as an indistinct blare. Phelps blended into the mix with the others  he was surrounded by other prospective Olympians, including Erik Vendt, Peter Vanderkaay and Allison Schmitt  and wore the kind of detached expression befitting someone who had done much the same thing most of the days of his life. “You’ve got to understand how monotonous and boring our training is,” Bowman told me. “And hard.” On several occasions, I watched as Phelps tried to irk Bowman  as Bowman predicted he would  by tugging on the lane divider during kicking exercises. “Michael knows it’s one of the things I absolutely hate most,” Bowman said. “When he first started doing it, I went nuts, and it wasted our time. It’s just a way he has of asserting his independence from me  a stupid way. I’ve decided that for now I’m going to pick my battles and ignore it.”

After 40 minutes, Bowman had his swimmers increase their intensity to a “pink pace,” their heart rates rising to around 150 beats per minute. The swimmers grew more serious, and it was clear that many were focused on racing their teammates in adjacent lanes. Phelps seemed indifferent to the contest and allowed others to beat him to the wall. It wasn’t until Bowman called for a “red pace”  around 165 beats per minute  that Phelps, swimming freestyle, poured on his power and broke away from surrounding swimmers by several lengths. Afterward, he climbed out of the pool, grabbed his phone and, I surmised, began sending text messages. He looked no more winded than if he had been skimming the pool’s surface on an inflatable raft.

The crux of Phelps’s superiority  the quality that allows him to maintain the power and efficiency of his strokes when other swimmers begin to falter and that allows him to overpower competitors in the final lap of a race  is his endurance. Years of Bowman’s drills have developed and refined Phelps’s capacity to endure, but from the outset Phelps brought to the pool physiological attributes that place him at the limit for his species. Under aerobic strain, Phelps produces far less lactic acid than other athletes. There is considerable controversy over whether lactic acid itself is detrimental to athletic performace, but it is known that elevated levels of lactic acid are accompanied by a suite of other metabolic products  hydrogen ions, inorganic phosphates  that interfere with muscle contraction and are the stuff of ruinous fatigue. Phelps and other swimmers are continually tested to measure their blood lactate levels (the lower they are, the better a body’s state of aerobic conditioning) and to determine the rate at which they clear lactate from their systems, which indicates their ability to recover. Bowman, who is unapologetically secretive, won’t reveal Phelps’s results, but some data have leaked out over the years, and they have become the stuff of swimming lore. After he set a world record in the 100-meter butterfly in 2003, Phelps’s blood lactate level was an absurdly low 5.6 millimoles per liter of blood, one-half to one-third that of other elite swimmers. At competitions during which Phelps has raced multiple events in quick succession, he has been known to clear lactate from his blood while racing. He also has the ability to tolerate high levels of blood lactate  to swim at full strength and speed while carrying a load of lactate that would bring other athletes to a relative crawl. It’s part of Phelps’s recipe for flourishing beyond all reasonable expectations in water. Genadijus Sokolovas, an intense former pentathlete from Lithuania who is the sports-science director for USA Swimming, explained to me how important this is: “I recently calculated how much Michael is going to swim in Beijing. We don’t know his final schedule yet, but if it’s similar to Athens, the total time he’ll spend in the water  preliminaries, semifinals, finals, warm-ups, cool-downs  will be the equivalent of running eight or nine marathons over the course of the Olympic Games.”

Image THE SWIMMER AT HOME: Famously clumsy on land, Phelps has abundant mental and physical gifts for moving through water. Credit... Finlay MacKay

The next morning, I returned to the pool, where I found Sokolovas standing on the deck in front of a bank of electronic equipment, administering what he called a “swim-power test” to a specialist in the butterfly from Indiana University. The swimmer wore a belt around his waist from which a fishing line transmitted measurements of velocity and force 60 times each second; at the same time, a camera mounted on a track on the pool wall followed him down the lane and another camera filmed him from below. The apparatus, which Sokolovas developed, analyzes a swimmer’s effectiveness according to 25 to 30 different parameters and gives coaches a way to quantify the costs of a swimmer’s mechanical flaws. “Before this,” Sokolovas said, “we were just guessing  high elbow position is better than low elbow position, or pulling in the middle is better than pulling from the side. Now we can test any hypothesis.” (Bowman does not allow Sokolovas to share specific results of any testing he has performed on Phelps.) Sokolovas is a sporting objectivist and regards Phelps and other members of the mystical vanguard of athletes with an unsentimental gaze. “Working on his technique with Coach Bowman for so many years,” Sokolovas said, “definitely helped Phelps develop a very efficient way to generate his velocity curve on these graphs, and years of high-volume swimming really developed his aerobic conditioning. But,” he continued, “I’m pretty sure we have many more Michael Phelpses in this country.” Sokolovas noted, with some frustration, that the United States lacks the kind of organized system of talent identification that other countries use to start developing athletes in early childhood. “There are plenty of methods we can use to evaluate a child,” he said. “We can look at their biological parents and project their anthropometric parameters; we can evaluate their aerobic ability by testing their maximum oxygen output or measuring their aortas. We can test how fast they adapt to long-term training. It’s easy to evaluate, but instead we rely on the athlete coming to us, the way Michael Phelps came to Bob Bowman.”

In that case, I asked, how likely are we ever to see the likes of Phelps again? Sokolovas brushed aside my question. “I’m pretty sure his records will be broken in 5 to 10 years. The swimmers who are going to do this are already in the system.” Doesn’t human potential eventually hit a wall? “No,” he replied. “There is no point at which athletes can’t continue to improve. You can always do higher-intensity training, or maybe higher volumes. A swimmer can do more training on land; or more strength training in water, like swimming against resistances. You can improve your technique. You can improve your nutrition. Basically, I don’t see any limits in swim performance. We’ll never build the perfect swimmer. The records will go up and up.”