Experiences in researching the documentation of the intelligence (codename Ultra) produced by breaking Enigma at Government Code & Cypher School, Bletchley Park, 1939-45, are described. The major problems are identified and shown to lie in the obscurity of the associated processes, disguised as they were within general bureaucratic arrangements. This resulted in invisibility in both National Archives documents and publications in the field of military intelligence. This invisibility, along with difficulties in locating surviving veterans capable of illuminating such historical material as there is, demands detective work, serendipity and luck in addition to the normal skills of historical method in telling the story. (Contains 5 notes.)