The fail-safe key was first mentioned by Kelvin when he and Desmond were underneath the Swan Station at the system termination spot. Kelvin explains that "the incident" was electromagnetism and geologically unique. He goes on to add that there was a leak, so now the charge builds up, and every time they push the button, it discharges it before it gets too big. Desmond ponders why they are made to push the button if they can just turn a key to end it. To which Kelvin chuckles, adding an analogy of how it would be like pulling your finger out of a dam and letting it all blow instead.