Combined with the rain, the trenches became squalid hell holes and death traps, the latter especially during artillery bombardments where many men were at risk of being buried alive. The wounded often died of exposure and drowning at the bottom of waterfilled shell craters. German forces were ordered to construct deep 30 ft. bunkers as additional protection from shell fire. This rendered much allied preparatory artillery fire ineffective at destroying German positions prior to an assault as at the Battle of the Somme in 1916. Allied troops were permitted only shallow bunkers and of course the trenches for protection by their commanders, on the premise that this enhanced their mobility.
British troops knee deep in mud and water in a trench.
Canadian 10th Division troops in the mud of Passchendaele, Belgium, November 1914. Rain filled shell holes at Passchendaele, France, Canadian 2nd Division November 14, 1917. Horses and equipment mired in the mud of Passchendaele.