The diver had found the fibre optic cable lying on the seabed of the North Sea. He swam closer, until it was near enough to touch.

He reached out his hand. But someone could tell he was lurking there. Someone was watching.

“He stops and just touches the cable lightly, you clearly see the signal,” says Daniel Gerwig, global sales manager at AP Sensing, a German technology company. “The acoustic energy which travels through the fibre is basically disturbing our signal. We can measure this disturbance.”