Nuclear weapons tests are conducted to determine the effectiveness and explosive capability of nuclear weapons. The first nuclear weapon was detonated by the US in 1945, and the largest test was the Soviet Union's "Tsar Bomba" in 1961. Underground tests continued into the 1990s until treaties were signed banning further testing. The most recent test was conducted by North Korea in 2009. Tests are categorized based on their purpose, such as evaluating weapon designs or effects, safety experiments, and detecting other nations' tests.