A heavy wooden door opens onto a long flight of steps leading deep underground.

It is dark and cool in this large network of man-made tunnels, and we wrap scarves around our necks.

Tiny bats hug their wings close to their bodies and hang tight to the cave's roof.

Archaeologist Gilles Prilaux swings his torchlight onto the limestone walls and exclaims excitedly: "This is where the story starts."

These caves, underneath the rustic village of Naours, in northern France, are a part of Australia's military history.

In a vast network of chambers and tunnels, some of them dating back to the Middle Ages, are the preserved signatures of hundreds of Australian soldiers who left their mark here during World War I.

The Naours caves stretch for kilometres and are about 30 metres below ground. Hundreds of years ago they were used by local villagers to store their goods or hide from invaders.

At one point 3,000 people lived in this underground city, building their own chapels, bakeries and stables. Chimneys were routed up through cottages above so no-one would know there was a population below ground.

Australian soldiers marked the walls with their names, rank and hometown. ( ABC News: Lisa Millar )

By the late 19th century the famous caves had become a local tourist attraction, and during World War I Australian soldiers fighting in the trenches nearby would head there for sightseeing.

'Sometimes they'd be dead tomorrow'

Four years ago local man Mr Prilaux was studying the caves' ancient history when he suddenly wondered what the names on the walls meant.

"Here you can see regiment number, battalion and here you see Australia," he says, pointing to the signatures.

He has now realised the caves contain the signatures of as many as 2,000 Australians. Some of them are quite detailed, including not only battalions, but their home addresses, even their height and weight.

"Behind this signature I can discover a part of the life of this soldier," Mr Prilaux says.

"I know for a lot of these soldiers it was the last 'write' of their life.

"Sometimes they'd be dead tomorrow and they wouldn't have written to their family, but they wrote on these walls."

Australians are the dominant nationality among the graffiti artists.

"They're coming from the end of the world and they want to put on the wall a message: 'I am here today, what's tomorrow?'.

"It could be Pozieres, it could be Mouquet Farm," he says, naming frontline battlefields.

The signatures have aided historians in discovering more about the soldiers. ( ABC News: Lisa Millar )

Two soldiers have marked on the wall that in July 1916 they were lost for two hours.

Someone else has drawn a small pencil picture of a soldier wearing a slouch hat.

Among the markings: "C Spark (Blazes) Sydney", "C Hide, Perth", "AH Edmonds, Sydney".

Caves could again be a draw for Australians

The director of overseas projects for the Department of Veterans Affairs, Ian Fletcher, is still awed by the site, even on his eighth visit.

"We always talk about the gore of war, the fighting, the battles, the mincer of war, but this is a different story, a story of behind the lines, of what they did in those times off and the relief that they had," Mr Fletcher says.

The soldiers were taking respite from the trench warfare when they visited the caves.

A young private from Sydney, Allan Allsop, wrote in his diary that while stationed in nearby Vignacourt "… in the afternoon, a group of 10 of us went to the famous caves near Naours, where refugees were accustomed to hide in times of invasion. The caves contain about 300 rooms …".

It was the clue historians needed to understand why the soldiers were there — they were simply sightseeing.

Australian soldiers were the most dominant among the graffiti artists. ( ABC News: Lisa Millar )

Mr Fletcher says many Australians do not know the history of their ancestors and they should "just ask" — the slogan for a new campaign to encourage people to explore the past.

The priority now is to ensure the protection of this significant site.

The Australian Government is working with French officials and Naours Caves operators to protect the fragile graffiti. Over the weekend, Veteran Affairs Minister Darren Chester visited the caves and offered $30,000 towards preserving them.

The mayor of Naours, Jean-Michel Bouchy, says he hopes Australians will visit.

"The attendance is important to us but it's more important that Australians can come and discover these writings," he said.

"They're very rare and that's what's important — they're memories."