Modern Europe

Today, with the spring sun trying to burn through an early morning Belgian mist, it is hard to imagine that this innocuous looking potato field was once the hellish moonscape of the front line.

By the end of World War I life on the surface had become untenable. Tons of steel fell from the sky in a near continuous bombardment.

The Germans retreated into thick concrete pillboxes. The British dug further and further underground.

Today we often picture the British Tommy taking a drag from a cigarette leaning against the wall of his trench. In reality, especially around the Belgian town of Ypres, tens of thousands were living up to 40ft [13m] beneath the ground.

That is how far they had to dig to be safe from the German shelling.

Memories of Passchendaele

After the battle of Passchendaele in 1917, when a quarter of a million British and Commonwealth soldiers died as troops advanced the few miles to Passchendaele Ridge, a brigade headquarters was built here.

Within a week of its completion in 1918, the Germans swept through during their spring offensive and recaptured all the territory gained at such a cost a few months earlier.

They moved into the tunnels and extended them. As the war drew to a close they were finally abandoned as British troops advanced again in the autumn of 1918.

They were sealed and flooded and have remained buried and half forgotten ever since.

Race against time

Peter Barton, historian, author and expert in underground warfare, is hoping to be able to get back into the tunnels.

He said: "Those dugouts which we've explored before, everything was preserved, from the actual structure of the thing itself to blankets, to the wire on the bunks, to newspapers!"

But it is a race against time. Excavations in a nearby clay-pit mean the water that is preserving everything inside the tunnel system might be drained and the process of decay will begin.

Worse still, the tunnels might collapse.

So for two weeks he and a handful of others have been taking painstaking readings at the surface to try to map out the tunnels, using pegs and tape.

"It's a mixture of ancient and modern. We've used ground penetrating radar, which gave us a certain trace. And then we've used the ancient method of dowsing, and that matched exactly," said Peter Barton.

Nasty reminders

Helping him is Johann Van de Wall, a Belgian enthusiast who has successfully excavated smaller tunnels elsewhere.

"You can't believe that people were living there, they must have become mad living underground. They were like moles! It's unbelievable what they have built," he said.

A few feet beneath the surface the diggers reveal a layer of clay that is redder then the rest.

It is caused by the rust from the artillery and mortar shells, ammunition, helmets, rifles and bayonets that lie as a reminder of the horrors of the numerous battles fought here.

Within a few hours several shells were unearthed. In Britain this might have meant a call to the Bomb Squad and the evacuation of the local area.

But not here. It is impractical.

They are simply piled up in a corner of the field and from time to time the Belgian authorities come to take them away to be destroyed.

Secrets could remain

Digging for the tunnels is a dangerous and expensive business. They need to find an entrance, but so far have been unable to.

The farmer wants his field back in a few weeks to plant his next crop, which may mean they have to stop digging until the autumn.

It has been deeply frustrating, as Peter Barton explained: "The depth is a problem; to the roof of one of the tunnels it's 37ft, but we also know there were entrances, and it's finding them that's difficult."

Unless they find a way in, the secrets contained in these tunnels for 90 years will remain there.

The fear is that they could be destroyed before they are ever properly explored.

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