France has reopened a labyrinth of medieval quarries under the northern town of Arras which the British army converted into an underground hideout for 24,000 soldiers during World War I. The BBC's Emma Jane Kirby went to investigate.
Soldiers endured cold and damp before the hell of the trenches
Eighteen metres (60ft) underground and immediately I'm cold, slightly unnerved by the dimness of the light and very uncomfortable as the chalky ceilings constantly drip freezing water on my head.
Although these secret caves were a huge step up from the horror of the World War I trenches, this was no cushy billet either.
Used briefly as an air raid shelter in World War II, the caves were sealed and largely forgotten about. But now they are a museum.
After an hour wandering around the network, my clothes are damp and my teeth are chattering.
The 24,000 soldiers who were hiding here for eight days before the Battle of Arras in 1917 must have been chilled to the bone before they surfaced into the daylight to fight for their lives.
The traces of the former occupants are everywhere. Graffiti on every wall tells the tale of the frightened soldiers, chiselling their initials into the chalk, desperate to leave some mark of their existence.
A mystery lady was carved on one of the walls
I stop beside a rough love heart and can just make out the letters L and J and I imagine how, 91 years ago, J must have been nervously waiting for her sweetheart L to come home. The Battle of Arras was so bloody that it is almost inevitable he did not.
A little further along, an artistic soldier had painted a woman's face onto the wall. She is ethereally beautiful and Madonna-like - a loved wife, an adored mother or perhaps just a much-needed fantasy or a lucky charm?
The tunnels are wide and tall - 12m high in some places - which allowed the British army to create a highly sophisticated network.
There was an operating theatre and a hospital with 700 beds, there were cook houses, post boxes for the soldiers to write their letters home and even a light railway.
Throughout the 19km (12 miles) of interconnecting tunnels, each pillar was clearly marked with a number to help the soldiers find their way around.
Each cave section was also given a name by the engineers who helped create them. The southern part that makes up the museum is called Wellington, named, perhaps wistfully, by the homesick soldiers of the New Zealand Tunnelling Company.
Angela Convers, the museum tour guide, told me the Northern section (not open to the public) had been dug by Yorkshire miners.
"It's funny to think that hidden 20 metres under this little French town, and completely unknown to them, you have Glasgow, Edinburgh, Crewe and London!"
On Easter Sunday, the day before the Battle of Arras, the soldiers held a service to pray for courage. You can still see the melted wax on the pillars they used as a makeshift altar and a painting by a soldier of his comrades at prayer.
At 0530 the next morning they were marched to Exit 10 and told to climb the stairs to the outside world.
At the last moment they were ordered to leave their great coats behind to allow them greater freedom of movement. As the hatch lifted, the first thing the men would have seen was that it was snowing.
The briefest glimpse into Arras cemetery tells the story of what happened to most of those soldiers. During the six weeks of battle, the British army alone recorded 4,000 casualties per day.
In the daylight, I remembered a prayer of thanksgiving carved into the chalky wall of a tunnel by a soldier grateful not to be in the mud of the trenches: "Thanks be to God for providing us with this shelter from shells and bullets."
The tunnels gave only a temporary respite from the horrors of the Great War. The second the trap doors opened, the protection abruptly ended.