Most nations have signed an international treaty banning the testing of nuclear weapons. But nine key states with nuclear capabilities are preventing the ban from entering into force.
Those nine have yet to ratify the Comprehensive Nuclear-Test-Ban Treaty.
Among them, China, Egypt, Indonesia, Iran, Israel and the United States have signed the treaty. But North Korea, India and Pakistan have not.
The Nuclear-Test-Ban Treaty prohibits all nuclear testing everywhere on the planet - above and below the ground, in the air and underwater.
All 44 countries with nuclear technology capabilities (at the time of the treaty's final negotiations in 1996) must sign and ratify the treaty before it can take effect. Until then, the ban is merely a suggestion that countries not develop or test nuclear weapons.
Worldwide, 180 states have signed the treaty, of which 148 have actually ratified the ban. Fifteen countries have not signed the treaty.
While the Comprehensive Nuclear-Test-Ban Treaty was open for signature between 1945 and 1996, more than 2,000 nuclear tests were conducted.
The U.S conducted more than 1,000 tests, while the former Soviet Union held more than 700. France ran more than 200 nuclear tests, and the United Kingdom and China each conducted 45.
Three countries have broken the unofficial ban on nuclear tests since 1996. India and Pakistan each tested nuclear weapons in 1998, while North Korea tried out its nuclear capabilities in 2006 and 2009.