The Osterhagen Key was a device possessed by UNIT that triggered the Osterhagen Project, a set of 25 nuclear warheads that would rip the planet apart. Martha Jones was handed the key in 2009 by General Sanchez, during the Dalek Invasion of Earth. (TV: The Stolen Earth) The project was triggered at Osterhagen stations stationed throughout the Earth, the locations of which included Germany, China, Liberia and Argentina. UNIT protocols stated that at least 3 stations must be active before the project can be activated. (TV: Journey's End)
The Osterhagen Project was once activated in 2009, and Martha Jones was handed the Key to the Osterhagen Station in Germany. She established contact with Osterhagen Station 4 and 5, which were being operated by an unnamed Liberian man (refusing to give his name given what they were doing) and Anna Zhou, and decided to activate it. However, in the midst, knowing there was one more thing the Doctor would do, she contacted the Dalek Crucible and threatened Davros that she would activate the project, since Davros needed all 27 planets, and with one gone, his master plan would fail. Davros, not wanting to let go of Earth nor his plan, teleported Martha to the Crucible, and the project failed.
The Doctor, who was also present when Martha contacted Davros, was horrified by the very idea of this, questioning Martha's sanity in willing to use it and refused to believe it an option even if "the suffering of the human race is so great, so without hope". Following the defeat of the New Dalek Empire and the return of all 27 planets to their rightful places, he asked her to get rid of it, to "save the world one last time". (TV: Journey's End)