LONDON (AP) _ Guns rumble in the dark and tracer bullets cut across no man's land. Smoke hangs in the air, mingled with the smells of cordite and frying bacon.

''I thought I was back,'' Tommy Keele, a 96-year-old veteran of World War I, said at the opening last month of a reconstructed trench of the Western Front.

Then he added: ''The smell of death - that's not here.''

Keele was among guests invited to the unveiling of the Imperial War Museum's latest attempt to convey the atmosphere of war to generations that have never known it.

Britain is caught up in memories of World War II this summer. Keele, who fought in both wars, said his recollections of World War I, and the slaughter of the trenches, were the more vivid.

''Two of my mates were buried under us by a shell,'' he said. ''We dug them out, horribly mangled, and buried them along the trench. As I pulled one, he fell to pieces. The smell of death was all over me and I couldn't eat or drink for three days.''

A year ago, the Imperial War Museum opened a reconstruction of a World War II air raid shelter.

''Now we have gone farther back to what it was like for the infantry at the Battle of the Somme in 1916,'' said Dr. Alan Borg, the museum's director.

''We tried to get the trench as right as we could. We found the only machine left which could sew sandbags the way they were made in that war, because the method of sewing was changed in the 1920s. And we went to the old Somme battlefield in France and brought back two bags of earth to get the texture right for the sides of our trench.''

Baron Hermann von Richthofen, the West German ambassador and a descendant of the German air ace Manfred von Richthofen, the ''Red Baron,'' attended the opening ceremony.

The impact of a battlefield is created with life-size models of soldiers carrying real guns, bayonets and gas masks. There are even stuffed rats.

In the freezing early hours of an October morning, a soldier in a niche in the sandbagged wall fries bacon over a brazier while a recorded Yorkshire voice reads a genuine letter home.

''You asked what life is like in the trenches,'' the soldier wrote to his father. ''It is very boring most of the time. We sleep in the day as there's not much to be had at night. You share house and home with rats and burn lice out of the seat of your pants with candles.''

Two sentries light cigarettes. A Lewis gun fires tracers. A shell has just exploded, demolishing sandbags and pouring fumes into the narrow trench.

Artillery roars in the distance. Commands and cries ring out from a dawn attack going ''over the top.'' A soldier with head and chest wounds groans.

In a dugout lie relics from the Western Front: barbed wire cutters, gas masks, a rattle to warn of a poison gas attack, ammunition boxes, a sniper shield, a windup phonograph, canned milk and postcards.

Four periscopes show no man's land from 100 yards to 2 miles in a vista created from contemporary photographs.

Cyril Dennys, 93, fought in the 1917 Battle of Passchendaele. He recalled that 65,000 tons of shells fired in 17 days to soften up the enemy also destroyed the drainage system of the level Flemish plain.

The attack began and raged for weeks in pitiless rain; men drowned in the mud. Britain lost 300,000 dead.

Visitors leaving the trench see a display case containing the four penciled sheets of the last letter to his wife Lizzie from Pvt. Jack Mudd of the London Regiment, and the red poppy he sent home from a Flanders field.

He put nine crosses for kisses after his signature.

''God bless you and my children and may He soon send me back to those I love is the wish of your faithful husband, Jack,'' is the last sentence of the letter.

Mudd was killed at Ypres on Oct. 26, 1917.

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