Ninety years after the battle of Passchendaele, officially known as the third battle of Ypres, a group of enthusiasts is attempting to dig up some of the key trenches of World War I.

Across a flat, muddy Flanders landscape, a solitary figure is plodding along the furrows.

Geophysicist Malcolm Weale is a battlefield detective who specialises in uncovering history that has lain hidden for generations.

In this case, the ground beneath his feet shields secrets of World War I.

The farmland near the Belgian village of Zonnebeke was criss-crossed by the trenches that saw horrendous loss of life - the whining of Malcolm's equipment betraying the metal fragments of shells and equipment turned up by the ploughs every spring.

But Malcolm and the archaeologists who called him in are looking for one particular piece of history.

Somewhere nearby is a remnant of the hidden war - the shelters and deep bunkers that protected troops from the hail of explosive.