In 1999, David Keys argued that the world's climate had been catastrophically disrupted by a massive volcanic eruption. Ash from the eruption, he argued, seriously reduce the amounts of sunlight reaching the earth, leading to a prolonged period of cold weather and drought, with consequent outbreaks of plagues and the sudden decline of several civilizations in Europe, Asia and the Americas. Keys cited a Chinese chronicle from 535 which recorded the sound of thunder from the south-west in terms that suggested there had been no mere storm. He also noted a passage in the Javanese Pustaka Raja Purwa, a 19th century compendium of history and legend assembled by the court scholar Ranggawarsita suggesting that a massive volcanic eruption had separated Java from Sumatra. This eruption would also have accounted for a hiatus in the sequence of kingdoms in western Indonesia about that time. Recent geological research has suggested that such an eruption was possible, but the evidence remains slender.
Keys, David, Catastrophe: An Investigation into the Origins of the Modern World. London: Century, 1999.
Wohletz, Ken, ‘Were the Dark Ages Triggered by Volcano-Related Climate Changes in the 6th Century? (If so, was Krakatau volcano the culprit?)', http://www.ees1.lanl.gov/Wohletz/Krakatau.htm