of CO2, engineers have looked for alternatives to petroleum. Some manufacturers imagine one day cars might work like this model and run on hydrogen. Instead of an internal combustion engine, this car uses a fuel cell, a kind of battery, which combines hydrogen with oxygen to make electricity. The waste product is simply water. Prototype vehicles already exist which can run on hydrogen. The challenge to this vision of the future is where, exactly, will we find the hydrogen. JOHN B. HEYWOOD, Sloan Automotive Laboratory, MIT: Well, petroleum comes out of the ground. Coal comes out of the ground. Natural gas comes out of the ground. It's down in the ground, and we bring it out and use it. There's no hydrogen stored in the ground. We have to make it. NARRATOR: The good news is that you can extract hydrogen from natural gas. The bad news is that CO2 is released in the process. JOHN B. HEYWOOD: So if you take natural gas and you produce hydrogen, you've taken the hydrogens from the natural gas, the hydrogen atoms, but the carbon atom is going to come off as CO2. NARRATOR: To mitigate the greenhouse gas problem, the hydrogen would need to be made not from fossil fuels, but by splitting water molecules, using carbon-free energy. And it's hard to see how renewable sources like solar and wind can do this on the scale required, especially if they're expected to generate the world's electricity at the same time. RON PRINN,