The Government Code and Cypher School (GC & CS), the intelligence bureau responsible for interception and decryption of foreign transmissions, moved into the Park in 1938. The radio station constructed in the park for its use was given the codename "Station X" -- this term is often erroneously applied to the code-breaking efforts at Bletchley as a whole. Station X was soon moved south to to Whaddon Hall, to prevent any attention being drawn to the Bletchley site.
Early visitors described themselves as members of Captain Ridley's shooting party. Later, the code-name for the project was "ULTRA".
Among the famous mathematicians and cryptanalysts working there, perhaps the most influential and best-known was Alan Turing. In 1943, the special-purpose electronic computer Colossus was designed at Bletchley Park. This computer was used to crack the Lorenz cipher.
At the height of efforts it is thought that more than 10,000 people were working at Bletchley Park during the war.
The Bletchley Park effort was comparable in influence to other WW II-era technological efforts, such as the crytographic work at Arlington Hall, development of microwave radar at MIT's Radiation Lab and the Manhattan Project's development of nuclear weapons.
At the end of the war, much of the equipment used and its blueprints were destroyed by order of Churchill. Though thousands of people were involved in the decoding efforts, the participants remained silent for decades about what they had done during the war; it was only in the 1970s that the work at Bletchley Park was revealed to the general public.
The Bletchley Park trust has been founded to further the maintenance of the site as a museum devoted to the codebreakers.