The scientists extracting cores in Wadi Lawayni did not attempt to inject CO 2 into mantle rocks. Mantle rocks could be more potent than basalts because they contain three times as much reactive magnesium and calcium. Zahasky still sees an important advantage to storing CO 2 in basalts and mantle rocks, though. Clearly, questions need to be answered before mantle rocks can start making a dent in CO 2 emissions. Plenty of Oman’s mantle rocks are that hot today, but they are five to six kilometers below the surface.