In the control room of each of the United Kingdom’s four nuclear submarines is a safe. Inside the safe is another safe, and inside that is a handwritten letter from the prime minister to the submarine’s commander telling him what to do if a nuclear strike has destroyed the British state.

When a new prime minister takes office, his letters are destroyed unopened, so it’s not clear how extensive the instructions are. According to the 2009 BBC Radio 4 report The Human Button, they include these options:

• Retaliate with nuclear weapons.
• Do not retaliate with nuclear weapons.
• Use your own judgment.
• Place yourself under the command of the United States or Australia, if possible.

“In that letter,” wrote the Daily Mail in 2008, “Gordon Brown conveys the most awesome decision of his political career … and none of us is ever likely to know what he decided.”

(Thanks, Zach.)