Project Cloud Gap

Project Cloud Gap: Demonstrated Destruction of Nuclear Weapons  was a program run by the United States Department of Defense and the Arms Control and Disarmament Agency from 1963 to 1967 (or 1969, according to other sources ) whose purpose was to "test the technical feasibility of potential arms control and disarmament measures". Arms control agreements discussed between the United States and the Soviet Union would involve on-site inspections, and such techniques - which involved giant drilling rigs and helicopter overflights to detect secret underground testing - were field-tested by Cloud Gap. The program was abandoned after a helicopter crash during a mock inspection exercise killed several team members.

Cloud Gap's aborted work culminated in Field Test 34, "an extensive mock dismantlement exercise" which demonstrated two things: if any party to a treaty attempted to cheat, the risk of detection was significant, and the party that cooperated and allowed for on-site inspection would see "significant amounts of classified information be put at risk and invariably lost".