The mission to do what exactly, who authorized it, who was running it, how much did it cost, what were the gains, what was the point.
Edit: If you need to keep public things that secret, maybe you shouldn't be doing them.
Edit: If you need to keep public things that secret, maybe you shouldn't be doing them.
During the Cold War the Soviets lost a nuclear submarine in international waters. The Soviets couldn't find it, the US (through its network of acoustic listening stations in the oceans) could. There was a multi-year effort to recover the sub, which required an elaborate cover story and operational security.
If the Soviets knew what the recovery ship was doing, they would have blown it out of the water. All the secrecy was necessary to protect the mission.