Soviet Union detonated the Tsar Bomba (also referred to as Kuzkina mat) at Severny Island, part of Novaya Zemlya. It was a hydrogen bomb. Its yield was 58 megatons of TNT, 3900 times bigger than Hiroshima bomb and around 10 percent of the combined yield of all nuclear tests to date. The yield could be even bigger: 100 megatons. But the Russians were afraid of such explosion.