David Keys, Ken Wohletz, and others have postulated that a violent volcanic eruption, possibly of Krakatoa, in 535 may have been responsible for the global climate changes of 535-536. Keys explores what he believes to be the radical and far-ranging global effects of just such a putative 6th-century eruption in his book Catastrophe: An Investigation into the Origins of the Modern World. This eruption was believed to have been even more violent than Krakatoa's 1883 eruption, and also the one that created Krakatoa's original caldera, which resulted in the creation of Verlaten Island and Lang Island. However, there are other explanations for the climate change, including an eruption of Ilopango in El Salvador, in Central America.