Osterhagen Project

An Osterhagen key was a literal key used to activate a series of nuclear weapons aimed at strategic points beneath the Earth's crust. Once these devices were detonated, the Earth would theoretically rip itself apart. The various keys were held by UNIT branches around the world by the time of the New Dalek Empire's invasion of Earth.

Overview
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)

The purpose of the key is among UNIT's most closely guarded secrets. 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)

A given Osterhagen key could only be used at secret facilities, called "Osterhagen stations", scattered around the world. Such stations were known to exist in Germany, China, Liberia, and Argentina. 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.
 * ''Given the location of the other stations, it is reasonable to assume that the unseen fifth Osterhagen station was somewhere in North America.

At the time of the War in the Medusa Cascade, three stations were made active. Martha Jones, Anna Zhou and a Liberian Man who refused to give his name had all made it to their respective Osterhagen stations. Consensus was reached between the three to use their keys. However, Martha Jones delayed their use long enough to offer the Daleks a chance to surrender. The New Dalek Empire stopped the detonation of the Osterhagen key by transmatting Martha before she could insert her key.

After the War in the Medusa Cascade, the Doctor asked Martha to persuade UNIT to dismantle the "Osterhagen Project" altogether, to prevent them from being abused in the future. (DW: Journey's End)

History
Little is known of the history of the "Osterhagen Project". However, the 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. Given her age, this would suggest that it might be a program initiated by — or at least known to — the Brigadier. Indeed, the placement of the nuclear weapons at the Earth's crust could have been effected by "the Inferno", or technology based upon it (DW: Inferno).