Osterhagen Project

The Osterhagen Key was a device that was held by UNIT 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)

The Osterhagen Key consists of a number of activation devices that plug into secret facilities scattered around the world. When at least three UNIT agents holding Keys are at any three or more of these facilities, a detonator is primed which will then set off a series of 25 nuclear devices embedded in the Earth's crust. Once activated, the series of explosion would have destroyed the planet and was to be used only if Earth was facing terror in where death was preferable to further pain.

The New Dalek Empire stopped the detonation of the Osterhagen key by use a Transmat to abduct Martha Jones before she could activate the device. After the War in the Medusa Cascade, the Doctor advised Martha that they should destroy the key and the devices to prevent them from being abused in the future. (DW: Journey's End)