An unknown volcano that erupted so explosively nearly 200 years ago that it cooled Earth has been identified as Zavaritskii (also spelt Zavaritsky). It's on Simushir Island on the Kuril Islands archipelago, a region disputed between Russia and Japan. Zavaritskii's eruption spewed so much sulphur dioxide into the stratosphere that annual average temperatures in Northern Hemisphere dropped by 1°C.