Osterhagen Project

An Osterhagen (pronounced oster hah-gan) key was a literal key used to activate a series of nuclear war-heads placed at strategic points beneath the Earth's crust. This was called the Osterhagen Project.

Overview
The Osterhagen Project was a planetary self-destruct system. It was meant to be used only if humanity were suffering unbearably and no hope or help was ever coming. The Osterhagen system was a chain of twenty-five nuclear warheads placed in strategic points beneath the Earth's crust. If the key was used, the system would be activated and the Earth would be ripped apart.

Other information
Little is known of the origin of the "Osterhagen Project". However, the unnamed German woman who provided food for UNIT forces at Osterhagen Station 1, implied that the Project had been around since her youthful visit to London.

Jack Harkness of Torchwood 3 was able to learn the details of another top-secret UNIT initiative, Project Indigo, but he had no knowledge of the key. (DW: The Stolen Earth)

The people of Auros used the Osterhagen Principle to destroy their planet before being captures by the Daleks. (NSA: Prisoner of the Daleks)

Behind the scenes

 * "Osterhagen" is the name of an actual German village, but is also an anagram for "Earth's gone".
 * The concept greatly resembles the Outer Limits episode, "Dead Man's Switch".