Twenty years ago, the world's first Comprehensive Nuclear-Test-Ban Treaty (CTBT) of 1996, that prohibits nations from conducting any kind of nuclear tests, either for civilian or for military purposes, was approved by the United Nations. At that time, more than two-thirds of the General Assembly's members supported it. That number has now grown to 183. Although the support was strong, some doubted whether the treaty could actually be enforced. After all, what prevents a nation from signing the treaty and then secretly conducting underground nuclear tests?

Licorne nuclear test – French Polynesia, 1970

To prevent nations from violating the treaty, the United Nations decided —shortly after the treaty was approved—to embark on an ambitious engineering feat of creating a global network of sensors that could detect illegal nuclear tests. This global surveillance network is known as the International Monitoring System, and comprises of 337 monitoring facilities spread across the world that uses a variety of methods to detect evidence of nuclear testing. These include seismic, hydroacoustic, infrasound and radionuclide detecting stations. These are some of the most sensitive instruments on the planet. Over the years they have recorded not only nuclear tests but earthquakes, rocket launches, supersonic airplanes breaking the sound barrier, the songs of whales, meteors entering and exploding in the atmosphere, the sounds of icebergs breaking and much more. Like a giant sniffer dog, the International Monitoring System looks, listens, feels and sniffs for irregularities on the planet.