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The Royal Observer Corps (ROC) can trace its origins back to the Raid Reporting System established in 1925. Originally the main role of the Corps was the visual recognition and tracking of aircraft in the skies over the UK. With the advent of the Cold War, however, its focus shifted to one of providing information in the event of nuclear attack, enabling forecasts of radioactive fallout to be made.

In the 1960s a nationwide network of 1563 underground monitoring posts, arranged in 40 Groups, was completed. By 1973 the number of Groups had been reduced to 25, with each one controlling 30 to 40 posts. The UK was divided into five Sectors, with each Sector containing five Groups. The majority of the staff were part-time civilian volunteers.

As a result of a defence spending review in 1990, most of the monitoring-post personnel were stood down the following year, and the organisation was finally disbanded in 1995.

The underground posts were constructed from reinforced concrete and internally were 7 ft wide by 16 ft long by 7ft high and had a 20 ft access ladder. They comprised an entrance shaft, toilet and main observation room, and were designed to accommodate three observers. A water sump at the foot of the entrance shaft was drained using a hand-operated pump connected to a vertical pipe with an outlet on the outside of the access-hatch block. Fixtures in the observation room included a bench, cupboard and bed. Above ground there were two louvred air vents, one next to the entrance hatch and the other at the far end of the chamber.

The instruments used for detection and measurement were as follows.

Ground Zero Indicator (GZI) This was essentially four pinhole cameras, one facing each of the cardinal compass points, housed in a cylindrical unit and was intended to record the position of a nuclear explosion. The GZI drum was fixed to a mounting on the access-hatch block.

Bomb Power Indicator (BPI) The purpose of this was to record the peak pressure of the blast wave. A dial gauge inside the post was connected to a tube protruding above ground to which a baffle plate was attached.

Fixed Survey Meter (FSM) This instrument was for measuring the intensity of the radiation. A probe, connected to a meter inside the post, was raised up the inside of a pipe going above ground from the ceiling of the observation room.

Llangefni ROC post was in operation from 1960 to 1981.

More of the original fixtures of an observation room, including equipment bench, cupboard and bed, can be seen in the photographs taken at ROC Ceunant.

Royal Observer Corps (Wikipedia); Royal Observer Corps – Observer Post