What is "danger line"

Usage examples of "danger line".

The seismographs had been able to give only fifteen minutes' warning, but that had been long enough to get everyone above the danger line.

Although he had not got eyes at the back of his head, his alert instincts told him just exactly when Merry was crossing the danger line, and then changing the rope to his left hand, he whipped Merry's knife from his pocket and balanced it in the palm of his hand, so that the moonlight shone on the blade.

Rhodan quickly stepped across the danger line, making certain not to arouse the suspicions of the monitoring system which was registering now as before the presence of one person only inside its active range.

I traversed the enclosure wall and stopped in front of the red danger line.

The danger line was thirty minutes away and there was no time to lose.