Growing concerns about a possible nuclear war and other global threats have pushed forward the symbolic Doomsday Clock by 30 seconds - to just two minutes before midnight.
The Bulletin of the Atomic Scientists (BAS) said it had acted because the world was becoming "more dangerous".
The clock, created by the journal in 1947, is a metaphor for how close mankind is to destroying the Earth.
It is now the closest to the apocalypse it has been since 1953.
That was the year when the US and the Soviet Union tested hydrogen bombs.
Last year, the clock was also moved forward by 30 seconds.