.ON APRIL 2ND a gizmo resembling a snow maker on the deck of the USS Hornet, a nonexistent aircraft-carrier anchored in San Francisco Bay, started making a haze of salty spray particles. The scientific cooperation responsible for the task, the Marine Cloud Whitening (MCB) program, led by the University of Washington, is actually exploring whether such sprays may help to slow weather modification via a type of "geoengineering". What does this phrase way-- and also why is it so controversial?