"When the broken battered trenches, Are like the bloody butchers' benches, And the air is thick with stenches, carry on."
In the summer of 1914 soldiers had set out for a swift war, but after several mobile and extremely bloody months, rival armies decimated by artillery fire found themselves locked in a stalemate along the battle's Western Front.
To better protect themselves, soldiers burrowed a line of trenches that stretched for 700 kilometres (475 miles) from the North Sea through France to Switzerland, where both sides would dig in for three years, suffering crippling losses but making little headway.
The first trenches were hastily dug, often shallow holes, becoming deeper and more elaborate as the stalemate dragged on. They were dug in a zigzag so that if an enemy soldier got in he could not open fire along the whole length of the trench.
Fields of barbed wire surrounded the ditches, aimed at slowing any advance through "no man's land" -- the area between enemy lines which was sometimes only a few dozen metres.
The warren of trenches usually included a frontline, a reserve line and a support line which soldiers would rotate through, usually spending only days at a time at the front.
'The stench of those bodies haunts me still'
Either way, daily life in the unhygienic trenches was miserable.
Heavy rain flooded the trenches, needing to be scooped out at downtimes. Walls collapsed in a muddy mess which would freeze in the thick of winter and made moving around with heavy arms and equipments a terrible ordeal.
Toilets were improvised in the trench -- going outside left you open to attack -- and with no means to wash, and garbage and dead bodies piling up in and around the ditch, the stench could be overpowering.
"The rank stench of those bodies haunts me still, and I remember things I'd best forget," wrote English poet Siegfried Sassoon, who fought on the Western Front.
Soldiers also had to deal with the scourge of rapidly multiplying rats who would feed off these bodies as well as lice and flies.
If this wasn't enough to drive entrenched soldiers to the brink of sanity, the constant fear of death certainly was.
Barrages of artillery fire and explosions in the trenches and constant sniper fire gave rise to the term shell-shock, the trauma and mental breakdown suffered by many soldiers on the Western Front.
"I knew a simple soldier boy, who grinned at life in empty joy. In winter trenches, cowed and glum, with crumps and lice and lack of rum, he put a bullet through his brain. No one spoke of him again," wrote Sassoon in the grimly-entitled poem "Suicide in the trenches".
Food often arrived cold, while bread was frozen or mouldy. For French soldiers, it was their daily litre of red wine and cigarettes which helped ease them through the day.
Mornings for both sides began with a pre-dawn "stand to arms" where men would climb up on the fire-step, rifle and bayonet at the ready in case of a dawn raid.
After an inspection breakfast would be eaten during an unofficial truce between the two sides, one of several such tacit "live and let live" agreements on the frontline.
While the threat of danger was never far, attacks were most likely to take place at night so days were dedicated to repairing the trenches and other vital jobs.
In quiet times, soldiers passed the time tinkering with whatever they could lay their hands on -- wood, scrap fabric or bullet casings -- to cobble together lamps, pipes, jewels or even musical instruments.
Letter writing and reading was a lifeline, with 10 billion letters and packages exchanged during the conflict, although many tried to hide the horror of their daily lives from their loved ones.
Nighttime brought raids, transfers of supplies and movement of troops between the trenches.
"The air is loud with death, The dark air spurts with fire, The explosions ceaseless are," wrote another war poet, Isaac Rosenberg, killed in 1918, in the chilling "Dead Man's Dump".