Natural gas pros: Cleanest of fossil fuels. Fully natural-gas-fueled vehicles could reduce carbon monoxide exhaust by 70 percent, carbon dioxide by as much as 30 percent, and nitrogen oxides by 87 percent, according to the Natural Gas Vehicle Coalition. Virtually no particulates. Surface land used during drilling has shrunk as much as 75 percent since 1970s. Abundant, storable. cons: Exploration, production are expensive (average offshore well, $4.3 million). Expensive to liquefy. Some of untapped gas is on restricted lands. status: For space and water heating, supplies more than half of U.S. homes; for electricity, about 15 percent (i.e., third place, after coal and nuclear). As vehicular fuel, powers 100,000 vehicles on U.S. roads. Novel sources (e.g., shale) and price decontrol have increased supply and reliability, but distribution bottlenecks loom. forecast: Improved seismic techniques (to identify and map stores) and drilling technologies make extraction easier, especially offshore.