This is file photo dated July 1, 1916, shows British infantrymen occupying a shallow trench in a ruined landscape before an advance during the Battle of the Somme. One of the most vicious conflicts of World War I, the Battle of the Somme raged across northern France from July 1, 1916 and by its close on Nov. 18, millions of men had battled through the mud and blood of the treacherous trenches on both sides. On July 1 alone, more than 20,000 British soldiers were killed and up to 40,000 were wounded, making it the bloodiest day in the history of the British army. (AP Photo/PA, File) ** UNITED KINGDOM OUT NO SALES NO ARCHIVE ** (Anonymous/AP)

All wars end; even this war will some day end. . . . When the trenches are filled in, and the plough has gone over them, the ground will not long keep the look of war.

— John Masefield. “The Old Front Line”

At 7:28 a.m. on Friday, hundreds of people are expected to gather around an asteroid-sized crater in the French farm country north of Paris to hear a bugler sound the Last Post, Britain’s haunting salute to its war dead.

It was at that moment a century ago that the monumental World War I Battle of the Somme opened to the detonation of 30 tons of explosives that British tunnelers had secretly placed beneath the German lines.

Today the crater that resulted is, in a sense, ground zero for the rites marking the centennial of the start of the great battle near the river Somme, which killed 131,000 British soldiers and devastated the nation.

A “sacred shell hole,” historian William Philpott called it — a place of death for both British and Germans.

But its preservation is mainly due not to the governments of Britain or France but to the son of a London bus mechanic.

Richard Dunning, 72, of Guildford, owns the crater. He said he bought it on July 1, 1978, after reading about the place, visiting and fearing that it was about to be filled in and lost to posterity.

“I knew that it was such an iconic and valuable piece of Great War history, it had to be saved,” he said.

He won’t say what he paid for it and has never charged for access. He said he could never face the ghosts of soldiers killed there if he did.

“I couldn’t look them in the eye and say, ‘I charge people money to see the place where you bled to death,’ ” he said.

Dunning’s preservation of the site “is of massive importance,” said Martin Middlebrook, author of the classic military study “First Day on the Somme.”

He said in a telephone interview that he had tried three times to get some kind of official commendation in Britain for Dunning, but hasn’t.

The so-called Lochnagar Crater, named for a nearby trench in the British lines, was in 1978 a forgotten scar on the French countryside outside the village of La Boisselle.

It was a hole 70 feet deep and 300 feet across, then overgrown with weeds and bushes, that had been used as a motorbike track, Dunning said in a telephone interview.

The echoes of the blast, said to have been heard 200 miles away in London, had faded 62 years before. The initial British attack that morning had been a disaster. And the crater became both a death trap and a refuge.

Dunning, who said he is retired from the advertising business, was mesmerized by the crater and its story. He said he had first read about it in the book poet John Masefield wrote after touring the battlefield in 1917.

“One summer with its flowers will cover most of the ruin,” Masefield wrote. “In a few years’ time, when this war is a romance in memory, the soldier looking for his battlefield will find his marks gone.”

But the crater withstood the seasons, and the first time Dunning saw the place, “it got me,” he said.

“It is awesome and breathtaking,” he said. (It is so big it’s visible on satellite images.) “I went back as often as I could. Sometimes for a day. Sometimes for two or three days.”

The Battle of the Somme took place about halfway through the 1914-1918 war, which the United States entered in 1917.

British forces were to lead an attack on the German lines around the river to take pressure off the French, who were fighting the Germans at Verdun, 170 miles to the southeast.

It was to start July 1 with the Lochnagar detonation, and others nearby, followed by a massive British and French advance. (A similar tactic had been used in the American Civil War outside Petersburg, Va., in 1864.)

It would be the deadliest day in British military history.

The Lochnagar blast was tremendous, hurling chunks of earth thousands of feet into the air and buffeting airplanes sent aloft to observe.

“The trenches simply rocked like a boat,” remembered Pvt. Harry Baumber, a member of the “Grimsby Chums,” one of Britain’s tight-knit “Pals” battalions, mostly from the port town of Grimsby.

“We . . . looked on in awe as great pieces of earth as big as coal wagons were blasted skywards,” he wrote, according to author Peter Hart’s history of the battle. “A great geyser of mud, chalk and flame had risen and subsided before our gaze.”

But the explosion, and days of preparatory bombardment, had been largely ineffective. Many German defenders had taken cover far underground, survived and rushed out with their machine guns when the barrage stopped.

They set up on either side of the crater and opened fire when the British attacked.

The Chums, and others, were caught in the open and cut down.

“I noticed men falling thick and fast about me, and all the time the tremulous chatter of machine guns,” Baumber recalled. “It was akin to striding into a hailstorm and the further you went the less and less became your comrades.”

“It was simply a massacre,” he wrote, “an absolute bloody desolate shambles.”

Nineteen thousand British soldiers — including many from Grimsby — were killed on the first day of a battle that would go on until November, and go down in Britain as a national tragedy.

“We were two years in the making and 10 minutes in the destroying,” a veteran of the Leeds Pals battalion told Middlebrook, the historian.

Dunning said at first he was interested in purchasing only a small parcel of the crater. But when the owner offered to sell him the whole thing, he bought it.

“It would have disappeared if I hadn’t,” he said. “Bulldozers would have descended on it and it would have been gone in a week.”

On Oct. 31, 1998, visitors to the crater spotted what appeared to be human remains sticking out of the ground, according to news reports. Authorities were notified, and a skeleton and a soldier’s belongings were unearthed.

The remains were identified as those of Pvt. George Nugent, 28, of Newcastle. He had been part of the attack on July 1, 1916, and went missing in action.

On July 1, 2000, he was buried in a local cemetery, 84 years after he was killed.

For this year’s crater ceremony — one of numerous July 1 commemorations — Dunning said the bugler and bagpipers will play, and singers will perform the grim marching ditty “Hanging on the Old Barbed Wire.”

In addition, participants will blow trench whistles like the ones used as a signal to launch the attack — the shrill call for the men to leave the protection of the trenches and go “over the top.”

It’s “a seriously evocative sound,” Dunning said. “It does make the hairs go up on the back of your neck.”