If the pilot light goes out the driver has to visually check all the carriage doors.

BBC: Tube users 'told to check doors'

Yet the basic principle of the cables is shockingly simple: Light goes in on one shore of the ocean and comes out on the other.

WSJ: A Dive Into the Digital Deep

But consider what happens when the power goes out -- when, without warning, we lose artificial light.

CNN: When May 19 became the day of darkness