What were “Ultra” messages?

For most of the war, we had broken some of the Japanese naval codes. This often allowed us to know Japanese ship movements and we could deploy our submarines – and other warships – to attack. (Sometimes their messages were decoded too late for us to respond.) These messages were classified as top secret or “ultra” sensitive. If the Japanese learned that we had broken their codes, they would have changed their coding systems and we would have lost a significant advantage.