KEY POINTS:
It is early morning in northern France, and on the streets of Arras people yawn as they head to work and traders in the market in front of the city hall set up their stalls.
But deep below the city, a labyrinth of caverns and tunnels probe deep
into the chalky rock - and there, everyday human life swiftly becomes a memory.
The dankness and darkness become oppressive, leavened only by astonishment at how any man could live and work and fight in this netherworld.
Yet such was the existence of a remarkable band of Kiwis - the 446 men of the New Zealand Tunnelling Company, whose exploits on World War I's Western Front are being showcased by France in a museum 22m below ground.
The tunnellers were brought to France in a bid to smash the murderous gridlock of trench warfare. Allied commanders grappling with the barbed wire, machine-gun nests and concrete shelters set down by the Germans realised that the best chance of a breakthrough might come from underground - of dealing a blow to the enemy from below his very feet.
Arras, perched on the frontline, offered a golden opportunity. Underneath the battered Gothic city, and reaching deep into the region, was a cluster of chalk mines dug since medieval times.
The idea was to connect these caverns, turning them into a secret, sheltered base for an infantry division, then tunnel towards the German trenches and lay mines to blow them apart.
To do this, though, Army chiefs needed seasoned miners, men with hardened muscles, the ability to drive through crumbly chalk and remain calm in the face of rockfalls and artillery shells crashing overhead - and play a lethal game with German tunnellers.
They found this rare breed of men working the gold and coal in the South Island. The miners, most of them Maori, were recruited in 1915 and arrived in France in March 1916, becoming the first New Zealanders to reach the Western Front. They worked with British tunnelling companies, drawn from Welsh and Geordie coalminers.
Tough, left-leaning and not given to military discipline, the Maori caused occasional headaches for their commanding officer, Major J. E. Duigan.
"He later grumbled that he had '17 ex-secretaries of labour unions in the unit' as well as members of the 'Red' Federation of Labour," according to the official account by the New Zealand Ministry of Culture and Heritage.
When it came to doing the job, though, the tunnellers could not be beaten. Their first task was to foil the Germans, who were trying to lay mines under British trenches. With the trenches in some places just 200m apart, this called for locating the enemy tunnel with extreme precision, then swiftly but quietly digging beneath and laying explosives in order to destroy it.
"It is all a most uncanny business, and fraught with danger," Malcolm Ross, war correspondent with the NZ Forces, wrote back home in 1917.
If, using stethoscopes placed against the rock, you heard that the enemy had stopped digging and was starting to pack explosives and you weren't ready to detonate your own charge, you knew you had lost the race.
"Not only was mining hard, with constant sweat, dirt and headaches, it was nerve-shreddingly dangerous and terrifying," says Nigel Jones, a British writer on World War I. "Not only could there be 'ordinary' soil collapses, at any moment the enemy could explode a counter-mine and bury you alive."
Forty-one New Zealanders were killed and 151 were wounded as a result of German countermining.
Despite the toll, the tunnellers won this battle of stealth and speed, forcing the German miners into a retreat from which they never recovered.
After gaining the upper hand, the New Zealanders carved through 7km of rock to connect up the ancient quarries, forming a network that ran for 24km and reached into no-man's land to just a few metres before the German trenches.
To help navigation, the caverns were named after New Zealand towns, running from north to south, starting with Russell and ending with Bluff.
The network became an underground city, capable of quartering an entire division of 20,000 men, complete with running water, electric light, latrines, kitchens, a small hospital with operating theatre and a mini-railway.
On "Zero-Day", April 9, 1917, the Battle of Arras began, launched with the detonation of three mines laid under the German line. Fifteen thousand infantrymen poured through the tunnels. In the ensuing fight, Canadian troops seized the strategic Vimy Ridge and German lines were pushed back 11km.
For decades, the tunnel complex was sealed off and the documents about it taken back to Britain. It was rediscovered in 1990, thanks to amateur archaeologists who have campaigned hard for at least part of it to be opened to the public.
The €4.5 million ($9 million) museum, to be inaugurated today, is called the Carriere Wellington (Wellington Quarry). VIPs at the ceremony include tunneller Ernest Norton's daughter Patricia Philson, 82, grandson Rod and great-grandson Matthew.
"For me to see what my great-grandfather was part of back in World War I and see the conditions they were working in will be special," said Matthew Philson, from Auckland. "It will be a special thing to witness and be part of, and touching for everybody."
Visitors watch a film about the tunnellers and take a glass lift to the cavern floor, where they are taken on a guided tour of 350m of passageways.
To be there is to glimpse the toil and camaraderie but also the bleakness and loneliness of living mole-like on the other side of the world.
"To Circular Trench", reads one ancient sign. Beneath it, without apparent irony, another sign says, "No exit".
A carefully preserved piece of graffiti has sweethearts sketched in pencil - young, fresh-faced girls with long hair, who must now be long dead.
A pile of rusting tins of bully beef and other food gives an indication of the miners' staple diet.
In Waitomo cave, the tunnellers stored wooden props. "Housekeeper required", a sign says, chirpily. In another part of the quarry, an unknown hand has carved a silver fern emblazoned with the words "Kia ora".
"It was a world war, but this was also the war for the city of Arras, so it is local history," said Arras city official Jean-Marie Prestaux. "We also opened this to thank the Commonwealth soldiers who came so far."