A century hence, when historians try to pinpoint the birth of the hydrogen age, will they focus on two weary tennis players vegging out in a Vancouver hot tub? It is as good a peg as any. For on that summer day in 1989 at the Hollyburn Country Club, a peripatetic Canadian geophysicist persuaded a British Columbia official to help fund a farfetched idea: a municipal bus that would run not on gasoline or diesel fuel but on hydrogen, and spew from its tailpipe only a thin stream of pure water. "Can you get me a green photo op?" Geoffrey Ballard, the geophysicist, remembers his companion asking.

Four years and $4.2 million later, the magic bus was built. Scientists from Vancouver's Ballard Power Systems, a then fledgling company, joined Canadian officials in drinking from a fluted glass the clean emissions of the world's first fuel-cell vehicle, celebrating an event they hoped would herald a transportation revolution. Since then, auto companies and other investors have poured more than $1 billion into Ballard's outlandish notion, betting that the fuel cell--an electrochemical device that combines oxygen with hydrogen to generate electricity--can all but eliminate auto pollution. With bravado Ballard predicts that fuel-cell cars will become economical by 2010 and "the internal-combustion engine will go the way of the horse. It will be a curiosity to my grandchildren."

If Ballard, a trim 66-year-old with an unflinching gaze, sounds cocky, it may be because he has finally won respect at the end of a long and winding career. The son of an electrochemist from Niagara Falls, Ont., he crossed the border to earn a Ph.D. in geophysics from Washington University in St. Louis, Mo., and worked for the U.S. Army in specialties ranging from microwave communications to ice physics (he studied how to hide bomber refueling tanks in Greenland). After the 1974 energy crisis hit, he became head of the new Federal Energy Conservation Research office in Washington but was frustrated when Congress refused to get serious about weaning the U.S. from imported oil. "So I quit," he says. "I've never followed the herd." His first business venture, a seven-year quest to build a lithium-based "superbattery" that would replace the internal-combustion engine, never panned out, landing him at one point in bankruptcy.