The eruption of Tonga's Hunga Tonga-Hunga Ha'apai early last year chewed up about 5-percent of ozone over the tropical south-western Pacific and Indian Ocean.
The volcanic plume, which reached 57-kilometres high, was full of ash, gasses, and - crucially - water vapour.
Scientists believe that water vapour cooled the stratosphere, creating the conditions for ozone depletion.
University of Canterbury atmospheric chemist Laura Revell spoke to Ingrid Hipkiss.
