LONDON (Reuters) - Cannon balls retrieved from ancient shipwrecks are wreaking thunderous havoc centuries later -- by exploding on the desks of archaeologists, New Scientist magazine said Wednesday. Robert Child, of the National Museums and Galleries of Wales in Cardiff, told the magazine that he had recorded several instances where the old rusted balls of metal started to heat up and turn red, or explode after being exposed to oxygen. In one case, a ball retrieved from a 1691 wreck heated up to a few hundred degrees after several minutes in the open air, began to glow a dull red...