To protect power station workers from this invisible threat the reactor core has to be encased behind many metres of thick concrete.

He shook his head and made a movement with his hand, as if warding off some invisible threat.

But the Order is still an invisible threat.

Nightly routine is a victim of the invisible threat.

They looked at the jungle wall with obvious fear, pointing their death-sticks at invisible threats.

The exercise was good, yes, but it also kept his mind away from the invisible threat above.

Grayson was its own people's worst enemy, the invisible threat perpetually waiting to destroy the unwary.

You feel this sort of constant invisible threat.

Often the dancers appeared to be battling invisible threats.

He seemed to be in no danger whatsoever, and that made him jumpy because he imagined invisible threats.