Workmen were laying a gas pipeline on a street in Arras, northern France, when they came across a flight of stairs going deep under the ground.

A tunnel carved into the chalk led to a British World War I field hospital. It had lain undiscovered since it was abandoned in 1917.

A network of caves was built to take in 700 casualties, and was just 800 metres from the German frontline.

A visit to the Hôtel de Ville's interior is none the less essential, because it is from here that the visitor can descend into the other, subterranean world of Arras, called Les Boves. There have been networks of underground galleries and caverns in Arras since the 10th century because - as in Paris - the stone for the town's buildings was quarried from the ground directly under its citizens' feet. In peacetime, the caverns produced by mining became storage cellars, particularly for wine; in times of war and revolution, they became useful hiding places.

The original tunnel networks were linked and hugely expanded by the British during the First World War. At the start of the war, the British Army had no specific organisation for sapping (trench-digging), mining or tunnelling operations - such work was largely relevant only to sieges. But on December 20, 1914 the Germans exploded 10 mines, planted under British positions by their sappers, at Givenchy. By January 1915, it became clear that systematic German mining was under way, and the British responded by forming eight Tunnelling Companies the next month. Men with suitable experience - as London Underground excavators, miners or sewer workers - were recruited, and a further 13 companies were later formed; these were joined by seven more specialist units from Canada, Australia and New Zealand.

Much of the sappers' tunnelling was carried out beneath open battlefields, but at Arras - thanks to the head start provided by the existing cellars and passageways of the town centre and another large network of mining tunnels in the eastern suburbs - the British were able to develop a vast underground facility, unbeknown to the Germans, with direct connections to the German lines on the eastern edge of Arras.

While German shells rained on the town above them, sappers chiselled out stairways, chambers and passageways in the style of pit-galleries, with timber props, good headroom and space for two to walk abreast. Electric lighting was installed to illuminate a command post, telephone exchange, hospital, chapel, kitchens and sanitary facilities. By the time of the build-up to the Battle of Arras in April/May 1917, underground Arras could accommodate 24,000 men.

For all but the most claustrophobic, life underground must have been infinitely more pleasant than life in the open battlefield trenches. Les Boves are relatively dry, with a year-round temperature of 52F, and enough natural light penetrating here and there (thanks to mining shafts) for delicate ferns and grasses to grow in niches. There were reminders of home in the names of the tunnels, such as Manchester and Glasgow; and troops scratched graffiti - pictures and names - into the walls.

Eighteen laborious months of tunnel construction, extending east into open country outside Arras, and six weeks of moving troops into the tunnels finally culminated in just 40 minutes of surprise attack on the enemy at 5.30am on April 9, 1917. In the words of Sir Douglas Haig, "Closely following a tornado of our shell fire, our gallant infantry poured like a flood across the German lines, overwhelming the enemy's garrison. Within 40 minutes of the opening of the battle, practically the whole of the German front-line system on the front attacked had been stormed and taken." Although begun so well, the battle would slog on for another 38 days, with the highest casualty rate of the whole war (more than 4,000 a day), before the German line was finally pushed back to a point about 12 miles from Arras.

A reporter for The War Illustrated, on his November 1918 underground tour of "the secret city of Arras", began by descending through a cellar trapdoor in one house and emerging through the kitchen of another. He said: "What possibilities of romantic and criminal adventure may not arise from these cellar doors that lead into this weird and widespread city of the underworld!" He was, I suspect, more romantic than the Arrageois, who seem simply to have resumed using their cellars for wine storage, and now open a section of Les Boves to tourists (not all the network is safe). Every spring, the passageways and caverns are fancifully transformed by plant installations into "Le Jardin des Boves". The First World War troops would probably have approved such an affirmation of flourishing life, in a place where so many faced death.

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.

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.

...

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.