The 1917 assault on Passchendaele caused the most unspeakable suffering of the Great War, surpassing even the carnage of the Somme.
The rows of white tombstones in the Tyne Cot cemetery at the top of the battlefield mark the graves of 11,908 men and make up the largest Commonwealth cemetery in the world.
Eighty years ago yesterday, as the battle commenced, a barrage of ammunition hitherto unimagined in warfare was hurled upon men struggling to find shelter in the mud.
As the bombardment continued, the rain and the shells obliterated even the trenches themselves, leaving the bodies of those who died in them unrecognisable. The epitaph on many of the headstones is simply: "Known unto God."
The cemetery is also a memorial to the 34,984 whose bodies were never found, who simply disappeared into the mud.
The small ridge on which Passchendaele is set was Field Marshal Sir Douglas Haig's target for the final stage of what was officially called the third battle of Ypres. As they crawled through three miles of mud, towards the village, the British troops were cut down by a hail of bullets and shells. Some 35 men died for every metre gained.
Such was the quagmire that regular supplies to the front collapsed and machine guns and rifles were jammed with mud. The men in the shell holes of the front line were forced to fight hand to hand. Passchendaele was reached, more than three months later, on 6 November.
Eighty years on, some of the veterans have been returning to the battlefield to remember old comrades.
"Passchendaele was the worst of all," said Dick Trafford, 99, from Lancashire. "It was hell itself. At the Somme you either got wounded or you got killed.
"[At Passchendaele] you got killed, wounded, or you drowned in the sludge."