When NASA activities are planned, our first priority must remain to protect the public and uphold public trust. This trust is achieved by communication between government and the people it serves — a task not without challenge. Secrecy bred of competition and proprietary technology can threaten communication between industry and government points of contact. This creates a barrier to sharing essential safety information, hidden against some other perceived kind of risk. Such information, known to few but not all of those with the need to know, can be termed as an "unknown known." This is the story of a great disaster, the Halifax Harbour explosion of 1917, where a dangerous munitions cargo entered a busy port, unplanned for, and known to few but unknown to key risk owners. An outbound ship struck the explosives-laden French freighter, sparking the largest man-made detonation yet. The sheer devastation made casualty counts difficult: approximately 2,000 were dead and 9,000 injured. Modern emergency planning and relief efforts sprang from this tragic event, the first disaster given extensive investigative treatment that can help us plan better nearly a century later.