[Question] Has government or industry had greater past success in maintaining really powerful technological secrets?

Context: As part of our efforts of working on Open Questions the LessWrong team has been reaching out to various researchers we know and asked them about questions they would be interested in getting answered.

This question was given to us by Ryan Carey and some of the answers below are the result of us trying to answer the question for a day on a private LessWrong instance, copied over to allow other people to contribute and read what we wrote.