Hydrogen-fueled cars have been hailed as the future of transportation — clean, safe and propelled by a power source whose only by-products are air and water.

"Your main three drivers for developing hydrogen are energy independence, economic growth and environmental sustainability," said Patrick Serfass, technical and program development coordinator of the National Hydrogen Association (search).

The problem, critics say, is that the technology that makes the fuel of the future generates just as much pollution as the gasoline-powered vehicles we drive right now.

"We need to understand where it's going to come from," says Dr. Michael J. Prather, earth-systems science professor at the University of California at Irvine (search).

Extracting useful quantities of hydrogen (search) from water requires a massive amount of energy — energy that typically comes from burning oil or coal.

You can also get hydrogen from methane (search) — but once again, it takes a "dirty" fuel to create a "clean" one.

Another possible problem: Scientists call hydrogen a "leaky gas" that easily escapes from any container you put it in, potentially harming the environment.

"It is not a neutral gas," Prather said. "It actually does interact in the atmosphere. And in some sense it needs a better evaluation."

Click in the video box above to watch a report by FOX News' William La Jeunesse.