In 1973, Richard Nixon ordered his Attorney General to fire Archibald Cox, the special prosecutor who was investigating Watergate. The Attorney General refused and resigned, and then the Deputy Attorney General became Acting Attorney General and similarly refused and resigned, then the Solicitor General, Robert Bork, became Acting Attorney General, and finally agreed to fire Archibald Cox. (This would later hurt Bork when he was nominated to the Supreme Court.) This event is commonly known as the "Saturday Night Massacre."
Now history might be repeating itself. Attorney General Jeff Sessions recused himself from the Russia investigation, so Deputy Attorney General Rod Rosenstein is now the Acting Attorney General for the purposes of the Russia investigation. But now that special counsel Robert Mueller is investigating President Trump for Obstruction of Justice, there are media reports that Rosenstein is considering recusing himself as well, since he was involved in the Comey firing which may be part of the investigation. And there is speculation that President Trump may fire Mueller, Rosenstein, or both.
So my question is, what is the order of succession in the Department of Justice? How deep does it go beyond Rod Rosenstein?