It's 7 p.m. on a warm, made-for-convertibles summer evening in Los Angeles, but Internet entrepreneur Elon Musk is still at the office. Not that he doesn't have better things to do. A million-dollar McLaren F1 sports car sits idle in his Bel Air garage. And with a pilot's license and a vintage Russian fighter jet in a nearby hangar, he could be doing loop-de-loops above Malibu right now. More important, his wife, Justine, had triplets two days ago, and they already have two-year-old twins—he really should be at home. But the round-faced, boyish 35-year-old, who has a net worth of $328 million and is ranked 12th on Fortune's 40 Richest Under 40 list, is hunkered down in his modest cubicle at SpaceX, the rocket-launch company he started in 2002, trying to explain to me why everything he's done in his young life—including cofounding the online payment service PayPal, in 1999—has simply been a preamble to this moment.

"Oil has humanity's head in a tightening vise," he says, glancing at the photo above his desk of Muhammad Ali, enraged and glowering midfight. The image seems to provoke him, serving as a reminder that anger is sometimes necessary. "There's the escalating price and its ability to destabilize the world, but most of all there's a limited window of time to reverse its impact on our climate. We need to move away from it right now."

When online-auction giant eBay bought PayPal for $1.5 billion in October 2002, Musk had the means to act on his

"Clean tech will be the biggest growth sector of the century," Musk says. "It's way bigger than the internet."

outrage. He saw his wealth not as a ticket to retirement but as a way to help build a new economy—one fueled on alternative energy and based on the promise that eco-friendly technology, or clean tech, can help solve the world's environmental problems. In March 2004, he invested nearly $6 million to start an electric-car company, Tesla Motors, in San Carlos, California. Soon after, he founded SolarCity, a photovoltaics company that aims to make solar technology affordable and accessible to the masses.

"Clean tech will be the biggest growth sector of the 21st century," Musk says, leaning toward me. "Put it this way: Energy is way bigger than the Internet, and almost all of it will have to convert to renewable generation. That's a big deal."

Musk isn't the only dot-com-era leader to bet on clean tech's potential. Google founders Larry Page and Sergey Brin have come aboard as investors in Tesla Motors, as has former eBay head Jeff Skoll. Page, Brin, and Skoll, along with venture capitalists like Vinod Khosla, a cofounder of Sun Microsystems, and John Doerr, the backer of home runs like Compaq, Netscape, and Amazon, have all invested heavily in the sector, which includes innovative companies in biodiesel, wind power, water purification, and recycling.

"I think a Google or Yahoo will be built in this area," predicts Khosla, whose San Francisco venture-capital company, Khosla Ventures, focuses almost exclusively on ethanol. "The next decade will be huge."

Indeed. In 2000, the global clean-tech sector had a market capitalization—or total stock-market value—of $7 billion; today, it's nearly $25 billion. As of August 2006, the WilderHill Clean Energy Index, a Dow Jones–type barometer that monitors the value of stocks in the clean-tech world, was up 50 percent since its inception in 2004. (The S&P 500 posted a 23 percent increase over the same period.) What Musk and his deep-pocketed cohorts hope to demonstrate is that we can't afford not to explore the clean-tech sector. Because there are, after all, two kinds of green.