Second World War 'brain' bunker hidden in the cliffs of Dover opens to the public

It has been kept secret for decades, buried four metres deep into the white cliffs of Dover. But from Friday, members of the public will finally be able to visit a pioneering command bunker that was ordered by Winston Churchill and proved a vital defence during the Second World War. Visitors will reach it via a hatch in a field on National Trust guided tours.

The Fan Bay plotting room, encased in concrete, housed the first land-based computer-aided targeting system. According to Jon Stewart, who led the excavation project for the National Trust, it acted as a "brain" for a gun battery that was used to defend Dover and attack German ships in the Channel.

Owing to its military importance, the bunker was secretly mothballed in 1957 and classified until 1981. Stewart was the first person to enter it when it was rediscovered in 2017. "It was a Tutankhamun moment for a conflict archaeologist," he said. "We had done all this research on the people who had served there, and then suddenly you're standing in their office. It was an exciting moment."

The plotting room and the artillery it controlled can be directly traced to Churchill. Soon after his "fight them on the beaches" speech, he visited the Kent coast and was alarmed to observe enemy ships passing through the strait of Dover unchallenged. At the same time, Dover was on the frontline of a Britain facing regular bombardment by enemy guns in occupied France.

In a July 1940 memo Churchill wrote: "We have to fight for command of the straits by artillery, to destroy the enemy batteries and fortify our own." He ordered the most advanced technology to be used and demanded weekly updates on progress on Dover's new defences. The guns were ready within 100 days and the plotting room bunker was completed 64 days later. "The guns were known around here as Churchill's pets," Stewart said.

Physical traces of the urgency of the construction are still visible at the site, Stewart said, pointing to the rough openings through the walls and floors for the cabling. Contemporary accounts describe how the equipment was installed before drawings governing its layout were available.

The plotting room housed a 1.8-metre-long electromechanical computer known as a fire direction table. It was used to calculate the position of moving ships at the moment shells hit and to send firing instructions to gun crews. Up to 10 people at a time worked in the room.

The computer was deliberately destroyed in 1957 to stop it falling into enemy hands. Stewart said the discovery reminded him of the rescue of Bletchley Park, the codebreaking site in Buckinghamshire, in the 1990s: unknown then, it is now part of the national consciousness, and the plotting room, he said, is a new hidden history. Against the film image of Wrens pushing markers around a map, he stressed that this is not a map at all but a really cutting-edge electromechanical computer, and completely unique.

The identities of only a handful of people who operated the computer are known. They include Wallas Eaton, an actor who later appeared as Senator Ludicrus Sextus in the TV comedy series Up Pompeii, alongside Frankie Howerd. A man named WP Bryant, a lance bombardier, was also recorded as a plotter at Fan Bay; he was a tailor, Stewart noted, so he probably had a head for numbers. What is known about how the machine worked is based on a manual and ghost markings on the floor, and Stewart is keen to hear from anyone whose family members served at Fan Bay, because the records are scant.

The plotting room has been preserved as Stewart and his team found it. Remnants include old newspapers found on the floor, old shoes and official notices. There is also chalked graffiti on one of the service tunnels saying "heroes fan hole Dodds and Petitt". Stewart said Dodds and Petitt may have been involved in installing cables to the bunker — probably self-appointed heroes responsible for marshalling horrible thick cable through particularly uncomfortable tunnels. Rather than recreate the past, the team has carefully conserved the room so visitors can experience it much as it was found, with all the marks of its history still visible.