What if there was an easy way to take the carbon dioxide from coal power smokestacks and turn it back into a rock that would sit quietly, deep below the earth's surface? That would get around a key sticking point of current carbon storage schemes, which entail injecting CO 2 into porous sedimentary rock formations such as sandstone—that the gas could eventually escape, seeping back up to the surface and into the atmosphere, heating the planet.

Basaltic rock, which makes up part of the earth's crust, could be an alternative to sedimentary structures. Minerals within basalt, including magnesium, calcium and iron, gradually react with CO 2 to form carbonate crystals inside the pores and seams of basalt, entombing the carbon as a permanent solid. This process, known as enhanced weathering, could capture massive amounts of CO 2 . Engineers are now trying to turn this bit of chemistry into practice.

This summer near Wallula, Wash., engineers injected almost 1,000 metric tons of CO 2 into layered basalt more than 800 meters belowground. For the next year they are monitoring how quickly and extensively those carbonate crystals appear. Some scientists have presumed that the process takes millennia to occur naturally, but laboratory results suggest it can occur in less than a decade. “It's not 1,000 years—it's not even several centuries,” says Pete McGrail, an environmental engineer at Pacific Northwest National Laboratory, which oversees the project. “We're talking a few years to a few decades to complete the mineralization.” That is quick enough to make a difference in the fight against global warming. Researchers expect to know more in December, when they have their first drill samples.

Engineers at a second project in Iceland, known as CarbFix, are injecting 1,500 tons of CO 2 over two years. They plan to pull samples in May and June 2014 and will continue monitoring through next December, according to Juerg Matter, a researcher at Columbia University who is involved with the work.

Some scientists are skeptical about whether the carbonate minerals are as leak-proof as hoped. Susan Hovorka, a geologist and carbon-sequestration expert at the University of Texas at Austin, says in certain conditions water deep below the surface could flow across the carbonate crystals and dissolve out the CO 2 , allowing the gas to possibly seep to the surface. Testing will be needed, she notes, to determine how well basalt will retain the carbon.