Top secret notes have been discovered at WW2 codebreaking base Bletchley Park -- used to line the unheated huts.
The notes were found in the roof of the now-famous base where British mathematicians and cryptologists raced to break German cyphers.
The notes were handwritten by Alan Turing's fellow codebreakers.
Turing and his colleagues worked in unheated huts, so they stuffed the walls with magazines and waste paper -- including these notes.
The codebreakers worked to decipher the encryption of the Enigma machine used by the Germans to encode messages.
Some of the notes pose a mystery even to today's experts, who can't work out what they mean.
Wartime security rules dictated that notes had to be destroyed.
As a result, this is the only surviving example of a Banbury Sheet, which was a method of deciphering German messages by comparing two messages with holes punched in them. By placing one message on top of the other and moving it until the holes lined up, the codebreakers could work out the daily settings of the German Enigma machine.
Mathematician, codebreaker and war hero Alan Turing.
The Government Code and Cypher School (GC&CS) recruited men and women for Bletchley Park based on their skills in languages, engineering, science, maths, puzzle-solving and even chess.
Today, Bletchley Park is a museum that has restored the 1940s base alongside new additions such as this statue of Alan Turing.
This is a re-creation of Alan Turing's office in Hut 8 at Bletchley Park. From this office, Turing directed efforts to break the German military's codes, presumably while shivering.
Though its uninsulated huts and blocks sound rudimentary, Bletchley Park was the world's first purpose-built computing center, built specifically to break the Enigma codes and help the Allies win World War II.
The Oscar-nominated film "The Imitation Game" dramatises the story of Alan Turing and his fellow codebreakers, although inexplicably makes no mention of how many jumpers they were wearing at the time.