Keys' book Catastrophe: An Investigation into the Origins of the Modern World (2000) theorizes that a natural disaster caused changes during the 6th and 7th centuries in the Christian world that included the end of ancient civilization and initiated what is now known as the medieval era. Key's work expands on that by Mike Baillie in Exodus to Arthur: catastrophic encounters with comets (1999) in which the primary evidence for the event is dendochronology (shown by narrow rings around c. 540 CE), as well as acidity in Greenlandic ice cores (suggesting volcanic activity), as well as accounts of historians such as John of Ephesus, Procopius, and Cassiodorus on changes in climate, including dimming of the sun. Keys adds further information from East Asia and develops his own version of the theory; he attributes the changes to a volcanic event in East Asia, in 535 AD, possible at Krakatoa.