When archaeologists discovered thousands of medieval skeletons in a mass burial pit in east London in the 1990s, they assumed they were 14th-century victims of the Black Death or the Great Famine of 1315-17.
Now they have been astonished by a more explosive explanation - a cataclysmic volcano which had erupted a century earlier, thousands of kilometres away in the tropics, and wrought havoc on medieval Britons.
Scientific evidence, including radiocarbon dating of the bones and geological data from across the globe, shows mass fatalities in the 13th century were caused by one of the largest volcanic eruptions of the last 10,000 years.
Such was the size of the eruption that its sulphurous gases would have released a stratospheric aerosol veil or dry fog that blocked out sunlight, altered atmospheric circulation patterns and cooled the Earth's surface. It caused crops to wither, bringing famine, pestilence and death.
Mass deaths required capacious burial pits, as recorded in contemporary accounts.