What is the 'Doomsday Clock' and what happens when it hits midnight?

The Doomsday Clock Is Now 85 Seconds to Midnight. Here’s What That Means

Doomsday clock set at 90 seconds to midnight

Doomsday clock set at 90 seconds to midnight

'Doomsday Clock' moves closer to midnight over threats from nuclear weapons, climate change and AI

7 Moments in the History of the Doomsday Clock | HISTORY

The Doomsday Clock, explained