The second-largest volcanic eruption of the 20th century, and by far the largest eruption to affect a densely populated area, occurred at Mount Pinatubo in the Philippines on June 15, 1991. The eruption produced high-speed avalanches of hot ash and gas, giant mudflows, and a cloud of volcanic ash hundreds of miles across. This animation shows the ozone measurements around Mt Pinatubo reacting to the eruption. That "hole" in the days immediately following the eruption of Pinatubo is due to interference by sulfur dioxide with the retrieval algorithm. There are high amounts of volcanic SO2 in the initial plume from the eruption. These measurements make it look like there was an ozone hole when there was not. Ozone really did decrease in the equatorial zone after the volcanic cloud spread throughout the equatorial zone over the next year or so. Plots of global average ozone show a clear minimum in the two years after the eruption. But that "hole" on June 20th for instance is sulfur dioxide, not an ozone hole.