These concealed bunkers, or Operational Bases (OBs), were dug out by Auxiliary Unit members, or in some cases created by the Royal Engineers. Accessed via a camouflaged entrance, they generally consisted of a corrugated-iron main chamber fitted out with bunks, a cooking stove and provisions to sustain a patrol for up to a month, as well as a smaller secondary chamber and an emergency escape tunnel.
Some were more elaborate, with chimneys incorporated into hollow tree trunks or spring-loaded entrance hatches designed to look like woodpiles, while others existed in disused mines or caves. In Kent, the architect of some of the most ingenious bunker designs was none other than Captain Peter Fleming, older brother of James Bond creator Ian Fleming.
Click here for the story on Britain's underground army in the Second World War.