During World War II, Bletchley Park was the site of the United Kingdom's efforts to break Axis ciphers, particularly the Enigma and Lorenz cyphers used by Nazi Germany. The codename for Bletchley Park was Station X; early visitors described themselves as members of Captain Ridley's shooting party.
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 cypher.
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 development of radar at MIT's Radiation Lab and the Manhattan Project's development of the atomic bomb.
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 maintainance of the site as a museum devoted to the codebreakers: see http://www.bletchleypark.org