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.
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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 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 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.
British WWI cave camp - Arras
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