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)
It was revealed that the Osterhagen key was a detonator that linked with three other users with similar keys at secret facilities around the world. These detonators were linked to a series of 25 nuclear devices that were 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)
Notes
Osterhagen is an anagram of Earth's gone. The is a reference that Earth has been stolen by the Daleks. The actual spelling of the name of the key is unknown, but it was pronounced differently during the episode The Stolen Earth by various characters.
