For distance runners, it's a mystical goal equivalent to the four-minute mile that Roger Bannister broke in 1954; a test so difficult some have estimated it may be 50 years or more before anyone achieves it.
Now the $270 billion market for athletic shoes and apparel is fueling an assault on the mark.
Nike and Adidas have announced separate plans to attack the barrier, with both introducing shoe lines linked to the effort. Wireless tech giant Vodafone last month said it was backing a third bid, hoping data gleaned from the quest will translate into wearable technology.
The leader so far is Nike, which came within a whisper of the barrier with a time of 2:00:25 in a run with top athletes on May 6. But Yannis Pitsiladis, a professor of sport and exercise science at the University of Brighton, England, may be the tortoise in this race with Nike's hare. He says running a 2-hour marathon will require a coordinated scientific effort and aims to raise $30 million for a project that will increase the understanding of the limits of human performance. Vodafone is backing Pitsiladis.
"Nike are doing this because they realized, 'Wow, I can sell more shoes,'" he said. "For me, this is not marketing. It's what humans can do when they work together. It's about human ambition, human legacy. It's like a journey to Mars."
Nike, which built its brand on the exploits of Michael Jordan and legions of runners wearing Nike's trademark Swoosh, is already selling shoes developed as part of its Breaking2 project. The Nike Zoom VaporFly 4% model features an aerodynamic heel and carbon fiber plate, which Nike says will make runners 4 percent more efficient than its previous top-line marathon shoe. The price: $250 a pair.
"Breaking2 is a quest to fully measure the extent of what the body is capable of," the company said by email. "Nike is looking to push the limits of human potential through product innovation, smarter training and an optimized environment — helping our athletes run what has never been run before."
The publicity generated by Nike's recent attempt, which was streamed live on the internet from a course festooned with red-and-white Swooshes, may have already helped the company reach millions of fitness fanatics and weekend athletes.
Running gear accounts for about 45 percent of the sporting goods market, according the market research firm Mintel.
"The running market is huge," said Samantha Dover, a retail analyst at Mintel. Runners "like to have goals, to measure performance, to record it for future motivation. The aspiration element is helping to drive the market."
And it's not just shoes. It's fabrics that wick sweat from your skin, fitness tracking apps and wristbands that monitor your speed. Millennials in particular are looking for gadgets that push the boundaries of wearable technology.
"Utilizing emerging technologies to enhance elite athlete performance in areas such as nutrition and training will unlock opportunities for these technologies to enter additional compelling markets, whether they be a broad consumer market or even one like the military," said David Carter, executive director of the Sports Business Institute and professor at the University of Southern California Marshall Business School.
Nike says it started the Breaking2 program three years ago.
First it selected three top athletes based on their oxygen use, energy consumption and ability to sustain speed over long distance. Initially, the program worked to refine their training by monitoring performance with GPS watches and heart monitors. Then it went a step further, measuring skin temperatures to determine the perfect race conditions and sweat rates to develop personalized hydration mixtures.
Finally, Nike took over the Monza racetrack in Italy for what was essentially a 26.2-mile experiment rather than a race.
The flat Formula One course with gentle curves provided ideal conditions for the three marathoners, who were supported by 32 other athletes who ran in groups of six to form a wind-breaking wedge and keep them on pace. Moped riders delivered their specially formulated drinks: No scrambling at the drink table at this race.
Eliud Kipchoge came agonizingly close to the goal, finishing in 2:00:25. One second a mile faster and he would have made history.
Although Kipchoge beat Dennis Kimetto's world record of 2:02:57, his time won't be recognized because the event wasn't sanctioned by track and field's governing body.
Critics say the Monza run may actually have shown how far humans are from running two hours in a sanctioned race. If some of the world's best athletes can't do it under ideal conditions, how long will it be before someone who has to contend with the vagaries of running through city streets in unpredictable weather breaks the barrier?
If the sub-two-hour marathon is the Mount Everest of human endurance, Kipchoge "has reached the ledge just beneath its summit," said Ross Tucker, the head scientist for World Rugby who writes for the Science of Sport blog. Creating perfect conditions just helped him get really close.
"It is, to finish the analogy, like getting so close to the summit with the benefit of extra oxygen," he said.
Pitsiladis, an anti-doping expert for the International Olympic Committee, is definitely not giving up. His star runner, Kenenisa Bekele, holds the second-best marathon time in history and finished second in this year's London Marathon.
Although Pitsiladis launched his SUB2 project in November 2014 with support from institutions such as Southern Methodist University, funding has been harder to come by because the effort was focused more on science than commerce. Pitsiladis says he has paid for some of the research with his own money, maxing out his credit card and mortgaging his house.
Vodafone recently became his first big name corporate supporter, offering technology to help runners and coaches track performance.
"A lot of people thought I was crazy," Pitsiladis said. "They're not saying this now."
Santiago Tenorio, Vodafone's head of network strategy and architecture, said the company's interest in SUB2 stems from previous advances it made working with high-end sports. The company once partnered with McLaren's Formula One team, and applied the technology it developed to serving its customers on highways. It wants to do the same with running.
He ticks off the things Vodafone will work on to help athletes, like offering real-time weather data or sensors that measure speed, pace, skin temperature and sweat composition.
"If we can help an athlete break the two-hour marathon," he says, "imagine what we can do with you."