Osterhagen Project

An Osterhagen key was a literal key used to activate a series of nuclear war-heads placed at strategic points beneath the Earth's crust.

Overview
The Osterhagen Project was essentially a planetary self-destruct system. It was designed to be used only if humanity was placed in a situation where their suffering was completely unbearable and no possibility for rescue or recovery existed. The purpose of the key is among UNIT's most closely guarded secrets.

A given Osterhagen key could only be used at secret facilities, called "Osterhagen stations", scattered around the world. Osterhagen Station 1 existed in Germany. China, Liberia, and Argentina had other stations. When at least three Osterhagen keys were used, a detonator would be primed which would then set off a series of 25 nuclear devices embedded in the Earth's crust. The power and positioning of the devices were such that the Earth's crust would be ripped open, effectively killing all life on the planet.

Threatened use
After Earth fell to the Daleks, General Sanchez of UNIT gave the key to Martha Jones along with the prototype of Project Indigo to escape. However, former Prime Minister Harriet Jones told Martha Jones not to use the key under any circumstances. (DW: The Stolen Earth)

Believing the Dalek invasion to be unstoppable, Martha Jones made her way to Germany and activated the station there and contacted two UNIT soliders, one Anna Zhou of China, another a Liberian who refused to give his name. UNIT protocol stated that the keys should be used immediately upon the activation of any three stations, but Martha Jones delayed their use long enough to offer the Daleks a chance to surrender. Upon discovering that the Doctor had been captured, Martha attempted to blackmail the Daleks into releasing him, but the Daleks transmatted her to the Crucible before she could insert the key, effectively disarming the system.

Martha Jones claimed that the Earth would be ripped apart, making the Reality Bomb useless (since it used Earth as part of its mechanism). After the War in the Medusa Cascade, the Doctor asked Martha to persuade UNIT to dismantle the "Osterhagen Project" altogether, to prevent it from being abused in the future. (DW: Journey's End)

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)

Behind the scenes
"Osterhagen" is an anagram of "Earth's gone".