It was the world's first electronic computer. Credited with shortening the Second World War, and thus saving countless lives.
Colossus whirred into action in February 1944.
70 years on, some of the codebreakers who worked it came to see it whir once more at the National Museum of Computing based at Bletchley Park, including Margaret Bullen.
– Margaret Bullen, Former codebreaker
"Milton Keynes wasn't here then, we were set in the fields.
We were just a little, tiny village called Bletchley, with fields all around us. It's unbelievable to think of it now.
I just feel very warm and proud of what I did."
As war raged in Europe, hundreds of codebreakers worked in secret to gain an advantage for the Allies.
Their task was to intercept and decipher German messages.
The Colossus was first used in February 1944 to crack messages sent by Adolf Hitler and his generals.
The work was invaluable, the processes complex, and it was all made possible by a computer big enough to fill a room.
The Colossus was 7ft tall and 17ft wide, and weighed a staggering five tonnes - more than an adult Elephant.
It also contained over four miles or wiring.
– Andy Clark, The National Museum of Computing
"The machines here (Bletchley), the Colossus in particular, helped speed up the codebreaking process.
The British military command and the Allied commands were on many occasions reading the German high command messages before the German commanders were reading them themselves."
The Bletchley veterans were sworn to secrecy for three decades after the war but now they have the memories of a job, and a machine to which they dedicated years of their lives.
Click below to watch a report from ITV News Anglia's Olivia Paterson