Ash particles from eruptions can be found in ice core layers.

Each year's snowfall shows up as a distinct layer in an ice core, meaning they can be counted like tree rings.

Visible layers in the ice core indicate periods when the summer snow melted and then refroze.

They did so by looking at thousands of years of river records and ice core deposits, all correlated with scrolls detailing uprisings and social unrest.

The effort took three years, but provided a continuous ice core that represented more than 120,000 years.