When there's an atomic explosion on Earth, the radioactive particles will drift up to this layer and set off a chain reaction. It'll go around the world in microseconds. And that's the end.

15 seconds sound clip from the Dimension X - The Outer Limit by Graham Doar classic radio play.

You can hear this line at 00:17:53 in the radio play.

Quote context

[...]

- What are they going to do?

- They've quarantined us!

- They've isolated the Earth, 'cause we don't know how to control ourselves yet. Until we learn, we'll be a menace to the whole universe!

- What is this nonsense?

- Quiet.

- How are they going to do it, Steve?

- They've spread a layer out here of - I don't know how to call it. All around the Earth - it's miles deep.

- When there's an atomic explosion on Earth, the radioactive particles will drift up to this layer and set off a chain reaction. It'll go around the world in microseconds. And that's the end.

- Yes? Yes. I understand. I've got to bring back the warning. You're going to put me back in my ship - to bring the warning.

- Now what?

- He's blacked out again. I guess that's all.

- What does all that mean?

- It's what he remembers.

- You don't think that really happened?

- No, no. The narcopsychometry circuits produce what he remembers. It just means that Steve believes this happened.

[...]