A labyrinth of tunnels, the Fan Bay Deep Shelter is reached by 125 steps down into the bowels of the white cliffs of Dover. It was built on Winston Churchill’s orders but abandoned and vandalised since the end of the second world war . On Monday it opens to the public for the first time.

The National Trust was astonished to discover three years ago it had inadvertently acquired the historic property, buried 23m below the stretch of cliffs overlooking the port which the charity bought through a £1.2m public appeal.

The tunnels, carved out of the chalk in just 100 days after a visit by Churchill, lie deeper than the nearby South Foreland lighthouse is high. Despite being stripped of metal sold off for salvage after the war, and suffering vandalism, including setting fire to the wooden supports of the longest 100ft tunnel – which then partially collapsed – most of it is in remarkably good condition, preserving graffiti including names of the men who dug the tunnels and those who later sheltered there, within earshot of the artillery on the French and Belgian coast.

The graffiti include what archaeologists call “latrinalia”, a cheerfully vulgar tradition dating back to Roman times and beyond. Although most of the toilet blocks were demolished in the 1970s, the bricks were used in rubble blocking the tunnels: inscriptions include initials, drawings, and the rhyme: “If you come into this hall, use the paper not this wall. If no paper can be found then run your arse along the ground.”

A more bitter message – a version of a newspaper headline criticising British military inertia, adopted by socialist sympathisers – was found on a piece of shuttering beside where a bunk bed once stood: “Russia bleeds while Britain Blancoes”.

One of the tunnellers has been traced through a pencilled note on steel shuttering: “Pioneer R A Foyle Compy, Thursday August 14 1941”. Reginald Arthur Foyle was born in 1912 and enlisted in the Pioneer Corps only a few months before his inscription. His company spent most of 1941 working underground at military installations along the south coast, but he transferred to the Royal Engineers in 1942, and his subsequent war career and later life have not been documented.

A book stashed for safekeeping on top of an air duct more than 70 years ago was also found: after conservation work it was revealed as a naval adventure story published in 1903, Shadow on the Quarter Deck by a Major W P Drury.

Other finds include quantities of homemade wire hooks, a Unity Pools football coupon for February 20 1943, British and American bullets, and a needle still with a trail of khaki wool tucked into the wall at the height of a top bunk bed.

The shelter was built along with a gun battery sited just 21.5 miles from France after Churchill visited Dover in July 1940 and was enraged to watch through binoculars enemy shipping moving freely in the English Channel. In a memo to the Joint Chief of Staff he said: “We must insist upon maintaining superior artillery positions on the Dover promontory, no matter what form of attack they are exposed to. We have to fight for command of the straits by artillery, to destroy the enemy batteries and fortify our own.” By 10 December the battery and still incomplete shelter were garrisoned by four officers and 118 men from the 203rd Coast Battery of the Royal Artillery relocated from Falmouth, and personally inspected by Churchill the following June.

The complex, which originally included five large chambers with storage space for rifles ominously sited between the bunks, a hospital and a secure store, a generator, and toilets and washrooms near the first world war sound mirrors, was carved out of the chalk by the Royal Engineers between November 20 and February 28.

Part of the complex, regarded as an eyesore and an attraction to vandals, was deliberately destroyed in the 1970s, when the original three entrance tunnels were collapsed and filled in with rubble.

They were rediscovered when a small hole in the ground was spotted, during work by the National Trust on the site. Experts from the Kent Underground Research Group burrowed into it, and found most of the tunnels had survived.

Dozens of volunteers came in to help clear the tunnels, removing more than 100 tonnes of spoil by hand, including 30 tonnes from the entrance staircase alone. They also carried in 80 railway sleepers to repair the tunnel supports.

Although the National Trust is reopening the site to small guided groups, a visit is not for the fainthearted. Visitors, the trust warns, must be aged over 12, “and in good health”, prepared for a 45-minute walk from the white cliffs visitor centre. The hard-hat descent, lit by handheld and head torches, is described as “an adventure in a dark, dirty and wet environment, and is not suitable for those who are claustrophobic or unsteady.”

Nevertheless the trust expects enormous interest in their latest attraction, and advises advance booking online. The trust would also welcome more information from the public about any of the men who worked on or served at Fan Bay.

