.ON APRIL 2ND a contraption resembling a snow machine on the deck of the USS Hornet, a nonexistent aircraft-carrier tied in San Francisco Bay, began producing a haze of salty aerosol bits. The clinical collaboration responsible for the project, the Marine Cloud Brightening (MCB) programme, led by the University of Washington, is looking into whether such sprays may help to slow weather modification via a kind of "geoengineering". What does this term mean-- and also why is it therefore controversial?