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After World War Two, some 30 Anti Aircraft Operation Rooms (AAOR) were built, largely hardened concrete structures. Each AAOR was designated to control a Gun Defence Area (GDA). In Northern Ireland there were two GDA's - Belfast and Londonderry. The AAOR at Lisburn was built, and later became the Headquarters of the No. 31 Group Royal Observer Corps area. There are no records in the Public Records Office about an AAOR built for the Londonderry GDA, however a few years ago this one was discovered a few miles outside Londonderry at Campsie, quarried into the side of the rock. Although the structure has been built the inside was never finished and fitted out, this can only be presumed as a cost saving measure, and ultimately the two Northern Irish GDA's were controlled by just the one AAOR at Lisburn, leaving the Campsie one abandoned.