What was the "Ultra" secret of World War II?
Ultra was the British intelligence designation of WWII. Ultra broke German, Italian and Japanese encrypted radio communications. The enemies believed their secret messages were indecipherable, but the Ultra project helped to draw the end of the European war nearer by cracking them.
The designation was based at the Government Code and Cypher School at Bletchley Park, and a small group of the best developers worked on it. They created a set of cipher machines, the most important ones were called Enigma and Tunny. Ultra's contribution in the Allied victory in WWII can hardly be overestimated. Anyway, the exact Ultra's influence on the course of war is still debatable.