By ROBERT HARDMAN

The wax is still melted on to the chalk pillar which served as an Easter Sunday altar for the men of the Suffolk Regiment more than 90 years ago.

Old helmets are scattered around the floor. A heap of cans, including a tin of Turnwrights Toffee Delight, lies alongside a collection of old stone jars - flagons of rum, perhaps, to numb the fear of the battle ahead.

The word "Latrine" is still written above an arrow on a 30ft chalk pillar. Next to it, two large rusting buckets sit beneath wooden holes.

Further down the labyrinth, another arrow points up to "No 10 Exit".

Here a staircase hacked into the rock leads up to a tunnel and on through 60ft of chalk towards the outside world.

Today, the tunnel is blocked. In 1917, it led to fresh air and daylight. But it was also a stairway to hell.

And I feel extraordinarily privileged to be one of the few people to climb it without feeling the angel of death sitting on his shoulders.

After the best part of a century, a stupendous remnant of World War I can be unveiled to the world.

Here, beneath the northern French town of Arras, years of careful excavation have finally unveiled the secret city where 25,000 British and Commonwealth soldiers lived just yards beneath an unsuspecting enemy.

Canteens, chapels, power stations, a light railway and even a fully functioning hospital were all established in this chilly labyrinth where I am standing with freezing water dripping on my head.

Scarred by the devastating losses on the Somme in 1916, British generals came up with a new strategy ahead of their next major offensive at Arras in 1917.

A series of subterranean medieval quarries on the edge of the town would be linked by tunnels to create the most extensive underground network in British military history.

These were not narrow shafts for men on all fours to crawl along. Tunnels had to be wide enough for soldiers to march in one direction and pass stretcher parties coming the other way. The larger routes had to accommodate a supply railway as well.

It proved to be a mighty feat of engineering but, in the chaotic aftermath of war, it was simply forgotten and covered up. But that neglect is our gain.

Today, much of it remains exactly as it was on that extraordinary morning in 1917 when, at the given signal, several British divisions burst forth under the noses of the enemy.

By the end of one day, they had advanced further into enemy territory than the entire British Army had advanced in years.

And yet the subsequent Battle of Arras would still see the worst bloodshed of the war.

As far as the Great War is concerned, the Arras discovery is on a par with the discovery of Tutankhamun's tomb.

Next to a suburban supermarket, beneath a former camp site, the public can take a glass elevator from the 21st century straight down to the world of Tommy Atkins and bully beef.

Clever lighting and sound effects have created a mesmerising insight into life on the Western Front.

Accompanied by a bilingual expert and an excellent audioguide, parties of 20 are able to weave their way through an authentic slice of the Great War.

And I have been allowed an exclusive wander among the chiselled walkways, wells and troughs, the 91-year-old graffiti and wall etchings.

Who is that mysterious dark-haired sweetheart drawn on the wall next to the regimental cookhouse? Who carved an exquisite little crucifix into a pillar?

The trenches, the poppy, the Somme and Flanders' fields have become sacred elements of our national identity - and that of many other countries.

The received story is one of heroic failure and senseless slaughter. We do not associate the Great War with much brilliance and ingenuity. But that was not the case in Arras.

The generals had learned a few lessons from the 1916 Battle of the Somme. Chief among them was the fact that frontal assaults on well-defended enemy trenches and artillery were mass suicide.

As the Western Front stalemate continued from the North Sea to the Swiss border, the French hatched a grand plan to win the war in 48 hours. They would smash through the German lines along the River Aisne in the spring of 1917.

The British would play their part with a colossal pre-emptive strike around Arras 50 miles to the north. A dazzling plan then took shape.

Today, Arras is an unremarkable town an hour's drive south of Calais. Most British tourists whizz past it on the autoroute as they drive to Paris and beyond. But if they look out of the window, they will glimpse some clues to the carnage in these parts.

Beautifully tended Commonwealth War Graves are dotted on either side. Soaring to the east is the stirring Canadian memorial to the 11,000 men who died in the heroic capture of Vimy Ridge. It is often said that Canada came of age as a nation that day.

Arras was a forlorn and battered frontier town. In 1914, it had been captured by the Germans, recaptured by the French and then put under British control to allow the French to concentrate elsewhere. In 1916, it was a shell of a place.

Civilians had been evacuated and British occupied the ruins while the Germans, who held the higher ground, sat to the East lobbing shells into the town.

It was just another stalemate situation on the Western Front. But, unseen by the Germans, something extraordinary was going on under the ground.