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

The Doomsday Clock is now at 90 seconds to midnight — the closest we have ever been to global catastrophe

What is the Doomsday clock? Why did it move closer to global catastrophe? What it means

Doomsday Clock moves closest to apocalypse than ever before, at 90 seconds to midnight

Doomsday Clock remains set to 90 seconds to midnight

The Doomsday Clock And Climate Change: What It Means For Humanity

The Doomsday Clock, explained

Doomsday Clock moves closer to midnight than any point since 1947