Mr. Combes, 35, and Mr. Russell, 37, both had collections of artifacts from observer corps posts when they began restoring the one in Cuckfield, chosen because it was in relatively good shape. “Some of them were flooded or had dead animals in them, or kids had been inside and set fire to them,” said Mr. Russell. With permission from the local council, they cleaned it up and stocked it with Cold War-era supplies, hardware and provisions, which were either donated by former observers, traded for with other collectors or bought on eBay, Mr. Combes said. In 2010 they began opening it for free public tours several weekends a year.

The post at Cuckfield is one of a handful the public can visit, but Subterranea Britannica (“Sub Brit,” to those in the know) organizes access for its more than 1,100 dues-paying members to various other underground and secret Cold War sites that are not generally accessible. Mr. Dixon, the club’s chairman, told me he enjoys underground sites from coal mines to transportation tunnels to military installations because, whatever their age, they are a good prism through which to study history. “Nothing is more exciting than going into a place that few others have seen or seeing something that’s been hidden for hundreds of years,” he said, adding, “and you’re never too far from the pub.”

One by one we descended the ladder to the cramped shaft leading to the post, taking care not to slip on the slick metal rungs. At about 100 square feet, the post made me feel as if I were in a rabbit hutch, albeit a large one. It had polystyrene panels covered in flame-retardant white paint on the walls and ceiling and was just big enough to hold the monitoring and communications equipment, including a giant gauge that measured radiation in kilopascals and a two-way land line “teletalk” system, which works sort of like an old-fashioned speakerphone. Mr. Combes and Mr. Russell sometimes fire it up to communicate with other restored posts. After a few minutes my feet became chilled by the cold, damp air that radiated from the concrete floor, over which scraps of carpet had been laid in a futile attempt to help insulate it.

After a nuclear blast, one of the three volunteer “observers” manning the post was to scamper out of the bunker into a likely radiation-contaminated environment, and remove the negatives from a pinhole camera adjacent to the hatch, from which they would be able to help determine the location and type of blast.

If a strike was imminent, Mr. Combes said, the observers would sound a hand-operated siren and fire small, flarelike rockets, indicating that people should “get underground or in your basement and stay there until these guys with their measuring equipment detected that the levels of fallout radiation had fallen enough that it was safe to come back above ground and carry on,” he said, making air quotes with his fingers around the word “safe.” Mr. Combes said he’s spoken with dozens of ex-observers, and while they are all proud of their service, some admit the system might have been ineffectual.

In addition to hardware, Mr. Combes and Mr. Russell have also provided ephemera like a copy of the Observer’s Handbook and the Official Secrets Acts, as well as decades-old cans of rations labeled “Curried Chicken,” “Steak and Kidney Pudding” and “Oatmeal Blocks” that reminded me of the bad-old days of uninspired British cuisine. Near the ladder, a small closet contains a chemical toilet and is stocked with vintage toilet paper, each sheet stamped “Government Property.”