Hydrogen is the most abundant gas in nature
Hydrogen is the most common element, but it doesn't exist in a ready gas form. It has to be split from oxygen. To expound on what Bob said I read recently read that it would take the total output of all the nuke plans we currently use to make enough hydrogen to replace *just* the petroleum we use in vehicles. And that's only a percentage of the total used in industry.
Not tossing water on hydrogen (huh-huh
), I would love for cars to be putting out water vapor instead of hydrocarbons, just pointing out that it isn't a magic bullet.