If you think heroism is an accident, you don’t know the Clown.

That was one of George Psychoundakis’ code names. Another was the Changeling, after those magical trolls who swap bodies. Yet another was "The Cretan Runner." It's this last name I kept coming across a decade ago while researching Born to Run—a mysterious Greek shepherd-turned-ultrarunner who become one of Word War II's unsung heroes. Years later, I finally had the chance to examine Psychoundakis' story in detail; I was so fascinated by what I found that I eventually travelled to Crete, researching his exploits for my next book project, Natural Born Heroes.

When World War II broke out, the Clown was a young and semi-goofy shepherd on the Greek island of Crete. Nobody thought of him as tough or especially brave; he was actually small and kind of skinny. If he was known for anything, it was for writing cornball poems, like his “Ode to an Inkspot on a Schoolteacher’s Skirt.” As with everyone else on Crete, he’d been lucky; the horrors sweeping across Europe hadn’t touched the island, leaving the Clown free to mosey along each day behind his flock. Until, early one morning, the sky went dark and an odd rumble cut through the coppery clong of the sheep bells. The Clown looked up and stared in awe as an airborne armada blocked out the sun. Hitler had decided he needed Crete and needed it bad; it was the perfect transit spot for his do-or-die assault on the Soviet Union, so he’d unleashed his elite airborne unit to conquer the island and crush even the thought of resistance.

And so, standing alone in a meadow, the Clown faced a choice: He could keep his mouth shut and put up his hands, or—with no warning, no training, and no weapons—go to war against the deadliest fighting force in human history. No one else in Europe had any trouble making that decision; after Hitler blasted through nine armies in a matter of weeks, not one country offered any spontaneous civilian resistance. None, that is, until Crete. While the Germans were still dropping from the clouds, Cretans were pouring out of their homes with axes and knives and ancient hunting rifles, banding with a ragtag crew of Allied soldiers to repel the invaders with such determination that they nearly delivered the Führer his first defeat. Once the battle was lost, the Clown took off for the wilderness and became a runner for the resistance, carrying messages some 50 miles back and forth between mountain hideouts.

Wait—was the Clown actually running on these missions? Yup. "I felt as if I were flying,” he’d say. “Running all the way from the top of the White Mountains to Mount Ida. So light and easy—just like drinking a cup of coffee." A British undercover operative described what it was like to have the Clown appear at a hideout late at night after one of his 50-mile scampers. “The job of a war-time runner in the Resistance Movement was the most exhausting and one of the most consistently dangerous,” he explained. The Clown would deliver his message, throw back a shot of moonshine—“A little petrol for the engine!”—and set right back off for his return journey. “We could see his small figure a mile away, moving across the next moonlit fold of the foothills of the White Mountains, bound for another fifty-mile journey,” the Brit recalled.

How is that even possible? How do you hammer out serial ultramarathons on a starvation diet, night after night, while dodging German patrols? For four years? The Clown wasn’t the only one, either. The island was crawling with these superathletes, I discovered—Cretans and Brits alike, all of them bounding across the peaks and bedeviling the Germans with ultra-endurance derring-do. So what did they know that the rest of us don’t? How could average people suddenly become unbreakable and thrive under challenges that would humble an Olympic athlete?

The answer was right there on Crete. For centuries, the island had been the quiet custodian of high-performance secrets of the ancient Greeks.

The Greeks didn’t just sit around hoping for heroes to appear—they built their own. They believed heroism was an art, not an accident, so they developed skills that were passed from parent to child and teacher to student. The art of the hero wasn’t about being brave; it was about being so competent that bravery wasn’t an issue. They learned to unleash the tremendous sources of strength, endurance, and agility that many people don’t realize they already have. Simply to survive, early humans had to be able to flow across the landscape: bending their bodies over and around any obstacle in their path, leaping without fear, and landing with precision. Heroes learned to tap into remarkable stores of reserved energy, all of it in their bodies—and yours—and waiting to be uncorked.

For thousands of years, the Greeks perfected the three pillars of the heroic arts—paidea (skill), arete (strength), and xenia (compassion)—and then they were gone. Luckily, their techniques still exist, scattered in bits and pieces around the world, some hidden right in front of us. Take performance fuel: As a professional ballerina, Leda Meredith was taught by her canny Greek grandmother that the best energy food in the world wasn’t just free; it was growing right under her feet. Likewise for Steve Maxwell: Brazilian jiu-jitsu fighters would travel to his small Philly gym for training advice because Maxwell was a student of sacred Hindu wrestlers and Golden Age boxers. “You never go wrong if you rely on the mighty men of yore,” Maxwell believes.

That’s the mission of Natural Born Heroes: to track down these custodians of the lost arts and revive the skills that can turn even a Clown into a hero.