Every scrap of metal on Earth was taken and used in making the rockets for the Master's war against the rest of the universe. There were in total two hundred thousand rockets. A hundred thousand were built in Russia. Each rocket had a black hole converter; if the Master so wished, he could activate them and destroy Earth. When the Tenth Doctor reversed time to prevent the Toclafane invasion, the rockets faded out of existence. (TV: Last of the Time Lords)