Comprehensive Nuclear-Test-Ban Treaty (CTBT)

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) bans nuclear explosions by everyone, everywhere: on the Earth's surface, in the atmosphere, underwater and underground.

Excerpt from CTBT website:

Why is the CTBT important?
It makes it very difficult for countries to develop nuclear bombs for the first time, or for countries that already have them, to make more powerful bombs. It also prevents the huge damage caused by radioactivity from nuclear explosions to humans, animals and plants.

Over 2000 nuclear explosions from 1945 to 1996

Over 2000 nuclear tests were carried out between 1945 and 1996, when the CTBT opened for signature: by the United States (1000+), the Soviet Union (700+), France (200+), the United Kingdom and China (45 each). Three countries have broken the de facto moratorium and tested nuclear weapons since 1996: India and Pakistan in 1998, and the Democratic People’s Republic of Korea (DPRK) in 2006, 2009, 2013, twice in 2016 (January and September) and 2017.