There was something very large buried under the spinach plot, that much was clear.
The infra-red imaging camera showed the ominous outline about 3ft below the surface, looming in the mud.
After an hour's digging by a dozen volunteers, it began to reveal itself. A metal hulk. A roof hatch.
Gun turrets... and caterpillar tracks.
Rising out of the earth was a unique relic of World War I ? a virtually intact British tank, interred on the battlefield where a German shell had halted it more than eight decades before.
The old lady who used to own the farm had told tales about it.
How German troops had ordered Russian prisoners to push the captured giant into a shell hole near a café owned by her parents in the village of Flesquieres, a few miles outside the French city of Cambrai, back in 1917.
But she'd never let anyone dig it up and it was only after she died, in 1998, that the lost tank Hudson kind of absolute secrecy that would not be seen again until the Enigma decoding machine in World War II.
The names it was given were deliberately vague. It began by being called HMLS Centipede ? HMLS standing for His Majesty's Landship.
An inter-departmental conference solemnly considered other names: Noah's Ark, Slug, Insect and Armouredillo.
Cistern and Reservoir were considered ? after all, what was it but a big metal box? ? but not Water Carrier because the initials would provoke smutty jokes.
Water Tank was thought better, Tank best of all. The order went out for 100 machines, each with a crew of ten.
None of the new Tank Detachment's officers and lower ranks was allowed to know what he was volunteering for.
Prospective candidates were simply told that "volunteers are required for an exceedingly dangerous and hazardous duty of a secret nature".
"Officers who have decorations for bravery and are experienced in the handling of men, and with an engineering background" were asked to attend Wellington Barracks.
Other ranks were enticed by advertisements in The Motor Cycle Magazine calling for men who could drive "light cars".
Months passed without a glimpse of the secret weapon.
And when tanks did begin arriving at the site, they were hardly a comfortable ride.
Ventilation was minimal. The petrol engine was in the middle, with the exhaust pipe going out the roof.
Either side was a narrow passage, with secondary gears and space for another gun at the rear.
Every inch of space was utilised, with cupboards for stores and ammunition cases.
There was no suspension: it was a bone-jarring ride and so noisy that the crew had to communicate by prearranged signals.
The side gunners in their turrets had little motorcycle seats on which to perch, but they could not stand upright.
The driver and commander had more room, sitting under a raised cupola with opening flaps, but during attacks they had to peer through tiny glass slits.
Late in the war a medical report itemised the health problems experienced by crews ? from skin rashes to burning of the nose and throat by fumes.
Held up: a British tank at the famous 1917 Cambrai offensive trapped in a German trench
Carbon monoxide poisoning picked them off. Behind them, on the single road leading to the front, machines sent to reinforce them formed a hapless traffic jam for enemy shells to destroy.
It was a vision of hell, lit by flames guttering from burning vehicles, fuelled by petrol and human flesh.
One officer described how "the holes in the paved way were filled with broken rifles, kit and corpses, the whole overlaid with stinking slime".
An engineer was sent forward to try to clear the way.
"As I neared the derelict tanks, the scene became truly appalling," he recalled.
"Wounded men lay drowned in the mud, others were stumbling and falling through exhaustion, others crawled and rested themselves up against the dead to raise themselves a little above the slush."
Seeking shelter, some soldiers had crawled into the nearest tank.
"Out of the doors protruded four pairs of legs ? the dead and the dying lay in a jumbled heap inside".
In the months that followed, losses mounted. The number of surviving tanks fell to a handful.
Word in London circles was that they were going to be put out of their misery, an experiment gone horribly wrong.
But the Army had invested too much money, and too much pride, to give up.
There had been no cause to imagine a weapon like this until the Germans started fighting from entrenched positions with barbed wire in front, making them almost impossible to overrun.
No one actually invented the tank, although Winston Churchill tried to take the credit.
It developed, painfully and unglamorously, through trial and error.
In 1914, when the war started, an Earl, four Lords, three MPs and the 2nd Duke of Westminster armed their limousines with machine guns, shipped them across the Channel and blazed away at the Hun.
The blue-bloods ran out of ammunition before red blood could be shed, but this idea of gun-armed fighting cars was a good one, and Lord Northcliffe patriotically donated the Daily Mail airship shed for developing an armoured car division.
As the British Expeditionary Force lost ground to the Germans, the War Office grasped at weird and wonderful solutions: high-powered water jets, giant explosive Catherine wheels and medieval-style sprung catapults to throw burning oil into enemy trenches.
One plan was for a fleet of armoured lawnmowers with wirecutters, powered by compressed air.
But one idea kept recurring: some sort of armed and fortified engine on caterpillar tracks to grip the ground. Unfortunately, no one had yet invented tracks that could bridge wide trenches without falling off.
Then, in September 1915, one William Tritton, the managing director of an obscure firm making agricultural machinery in Lincoln, found the solution ? a new kind of flanged track that would not droop away when suspended in thin air over a trench.
He also developed wheels at the back to steer it.
From the moment the green light was given for its development, the new machine was shrouded in the could rise again.
Busloads of tourists came to watch it being hauled from its grave like a mammoth from the ice.
There were no bodies inside, just bullet cases, corned beef tins and old bottles.
One of these particularly baffled the French investigators ? until it was identified as having held HP Sauce.
The tank had a name, too.
It was D.51, or Deborah (tanks, like warships, tend to be feminine). Army records show it was commanded by 2/Lt Frank Heap, the 23-year-old son of a Blackpool businessman.
Tall, bespectacled and with the looks of a country parson, Heap ordered his crew to evacuate after the tank was crippled by a barrage of fire.
Then a fatal shell struck ? cutting through the tank's armour and blowing up inside. Heap and three of his crew made it to safety.
The other four did not, either blown apart by the shell or cut down by machine-guns as they fled.
The Germans later buried the tank to use it as a defensive dug-out. Today, D.51 Deborah stands in a barn at Flesquieres, a solemn monument to all the young men of the British Tank Corps.
They are heroes whose story is littleknown ? human guinea pigs, who lived and died in a desperate experiment to break through German lines during the slaughter of the Great War.
As a fascinating new book reveals, few were professional soldiers.
They were motoring enthusiasts and mechanics, plumbers, circus performers and polar explorers ? a "band of brigands", in the words of one officer, trained in secrecy to pioneer Britain's secret weapon.
These slow-moving juggernauts, each weighing nearly 30 tons, were like nothing that had been seen before.
Their task was to crush and burn the enemy out of its fortifications on the Western Front.
Yet in their early days, they proved at best an embarrassment, at worst a disaster.
Sent into action at the start of the Flanders Offensive on July 31, 1917, a total of 77 tanks were simply swallowed up by the mud that oozed for mile upon mile across the battlefield.
As their crews struggled through the hatches like escaping submariners, German snipers
led to headaches, giddiness, palpitations, vomiting, unconsciousness and convulsions.
Despite all this, tanks worked. At least, they worked in trials on the Norfolk parklands of Lord Iveagh, spanning trenches and knocking trees over.
How they would stand up to shells or machine-gun bullets no one knew, because tanks were so precious no one could afford to test one to destruction.
Nevertheless, everyone was agreed they could take Britain to victory.
George V had a ride in one; he descended shakily, convinced they would be a great success. Feted like circus elephants, the first tranche arrived in France to be put through their paces.
When a German aeroplane flew overhead, they were hidden under tarpaulins painted to look like haystacks. Two issues remained unclear.
One was tactics.
In the words of a veteran: "There were no tactics when I joined; the only tactic was to get into your tank and drive towards the enemy; the infantry with any luck would follow you."
The other was strategy.
Everyone recognised that tanks needed to be deployed en masse to maximise the surprise as they reared over German trenches in the morning mist.
"None can be used until all can be used at once," advised Churchill.
But Douglas Haig, the British Commander in Chief, thought he knew better.
So when the very first tanks went into battle, on September 15, 1916, they did so dispersed in sections of six tanks or fewer.
Untried in battle, unable to communicate, unused to shell-holes, glugging petrol, vulnerable to machine and field guns, most of them ended up like beached whales, helpless before enemy fire, adding casualties to the bloodbath of the Somme.
Almost a third failed to reach the battlefield at all, because of mechanical faults. But it was early days, and the British public took the monsters to their hearts.
Once the secrecy was lifted, tanks became the subject of songs and caricatures.
Clockwork toy tanks were all the rage, along with tank tea-pots and tank piggy-banks.
Knowing how little they had achieved, the Tank Detachment hated the fuss.
The Arras offensive of March 1917 was a disaster.
New tanks were sent out with a shortage of spares and no armour plating, leaving their young crews at the mercy of German bullets.
After this, and the debacle at Passchendaele, when the tanks were again dribbled across the front line, the great secret weapon was in danger of being shelved.
But Haig was still inclined to give it another chance ? and this time he agreed to let the tanks be utilised in mass formation.
The German lines south and west of Cambrai were to be the objective of the first mass-mechanised armoured attack in military history.
It would be made against the strongest German defences on the Western Front.
The whole of the Tank Corps in France were to be employed ? nearly 500 machines ? plus 19 infantry divisions.
The tanks would advance in arrow formation ? one in front, two behind.
Shock was imperative, secrecy vital. Cooking fires were forbidden, even in daylight.
To the disgust of the men, even smoking was banned.
D-Day was November 20, 1917.
The tanks had been brought up by rail, and driven to their allotted assembly areas under camouflage.
Fuelled with whisky and rum (the war could not have been fought without liquor) the tanks idled forward, waiting for the artillery barrage on the German lines before picking up speed.
As one account put it, they flattened the enemy barbed wire like "beds of nettles".
A German writer recalled the terror that spread through his comrades' ranks as the tanks smashed forward through the gloom.
"Suddenly indistinct forms could be discerned. They were spitting fire and under their weight the strong and deep obstacle line was cracking like matchwood.
"Alarm! The troops rushed to their machine guns: it was all in vain! The tanks appeared, not one at a time, but in whole lines, kilometres in length."
Then the infantry swept in behind them, bayoneting the defenders without mercy. Within a few hours, the German lines had been overrun.
The critical objective, west of Cambrai, was Bourlon Wood. The tanks broke through to it and then, their fuel and ammunition exhausted, waited for the cavalry to take over.
In fact, the Cavalry Division was hardly off the start line, picking their way slowly through the German defences.
It was a gross failure of planning which would allow the Germans to collect themselves and hit back.
Some of the fiercest fighting was at the village of Flesquieres, where a succession of tanks were blown apart by German guns.
In the thick of the action was D.51 Deborah, with Frank Heap squinting through the observation pinhole.
Around him, other tanks were burning. Bullets and shell fragments were pinging off Deborah's hide.
Yet on they fought, smashing through a barricade and clattering down the village's main street to engage the enemy guns - until the direct hit that stopped them in their tracks. Heap was later awarded the Military Cross for his "great gallantry and skill".
Despite such losses, Cambrai was the victory Britain had been waiting for. Some 8,000 prisoners had been captured and more than 100 guns. The headlines were exultant: "Tanks Shock to Huns" rejoiced one newspaper.
War was far from over. But the role of tanks in warfare was established beyond all doubt.
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