A receiver control bay in Knockholt, Kent (Picture: Crown Copyright).
Government Communications Headquarters (GCHQ) is celebrating its centenary by revealing five former secret sites.
Dotted around the UK, unsuspecting locations used throughout history for wartime codebreaking and communications have now come to light for the first time.
Among the top-secret operations held at the former sites were efforts to decode Adolf Hitler's messages to his field marshals and the interception Soviet Union communications during the Cold War.
Here's what GCHQ revealed...
Ivy Farm - Knockolt, Kent
During the Second World War, this farmhouse hosted up to 100 staff from the Foreign Office Research and Development Establishment - which now resides in GCHQ's modern building in Cheltenham.
Technologists worked to decode what has now been dubbed 'noise' - sound that doesn't include human communication - carrying the secret to Hitler's most secure messages with his field marshals across Europe.
The 'noise' is said to have been far more secure than the infamous Enigma code, and cracking it led to the creation of the world's first computer, Colossus. Ivy Farm also witnessed the UK's first-ever fax interception.
Abbots Cliff House, Capel-le-Ferne, Kent
Between 1940 and 1945, this site collected Very High Frequency (VHF) communications from Germany.
Germany was using the signals to direct aircraft and E-boats in the English Channel.
Young, often female, German-speaking linguists were used to log the messages and protects British pilots and sailors.
Croft Spa, Scarborough
This countryside site was in use between 1940 and 1975 as a direction-finding station.
Croft Spa worked with other locations to pinpoint signals from enemy ships in the North Sea.
Located in the Scarborough countryside this site was operational between 1940- 75 and was home to up to 20 staff.
Nearby farm work would often have to be paused to allow for effective communications.
Marston Montgomery, Derbyshire
A series of wooden huts were operational here between 1941 and 1947.
Around 100 staff would listen to 25 radio signals, searching for enemy signals.
Once identified, the technical radio base would flag the information - which could then be used to identify enemy troops movements.
Chesterfield Street, Mayfair, London
From 1944 to 1953, this office block started GCHQ's Cold War operations.
Here, the first interceptions of the Soviet army, navy and air force took place.
One of ten to originally join, Bill Bonsall, went on to become GCHQ director in 1973.