The Bulletin of Atomic Scientists is an international association initially comprised of scientists involved in the Manhattan Project, the U.S. endeavor to create the atomic bomb. It now has atomic scientists, physicists and Nobel Prize winning scientists.
The use of the clock is intended to reflect basic changes in the level of continuous danger in which mankind lives in the nuclear age, with the countdown to midnight indicating how close we are to total destruction. It tracks dangers posed by climate change, cybertechnology, nuclear weapons, biotechnology, and military adaptations of new technologies.
The clock was recently (after Trump was elected) moved 30 seconds closer to midnight, from 3 minutes to 2:30 seconds, the closest it has been since the height of the Cold War. The closest was when the the clock was set at 2:00 minutes, when the U.S. conducted nuclear weapons tests in the 50's.
http://www.wired.co.uk/article/what-is-the-doomsday-clock
