The findings, published in the Proceedings of the National Academy of Sciences, point to the Zav-1 eruption from the Zavaritskii caldera on Simushir Island, a remote volcanic island northeast of Japan. One candidate—Ferdinandea, a short-lived submarine volcano off Sicily—erupted in July 1831, coinciding with the blue sun sightings. These microscopic fragments were chemically matched to pumice deposits on Simushir Island, specifically a thick layer of gray volcanic ash identified as the Zav-1 eruption. Location map of Zavaritskii caldera, Simushir Island, Kurils. Big Impact From a Forgotten VolcanoThe Zav-1 eruption is estimated to have released around 12 ± 3.5 teragrams of sulfur into the stratosphere.