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.
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.
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