Passchendaele is one of the original default maps of WW1:Source 1.13B. It is based on the military operation of the same name, taking place between June and November of 1917. The map is fairly tilted more towards the Central Powers, due to their better defensive position and close proximity to two capture points.
As one of the beta maps, it was subject to much gameplay, which gained it a reputation for being a good realism map. It eventually got a good touch up by 3rd Marines founder and former leader A. Switz, in his Passchendaele remake. The no-man's trench is a large ditch, with flat land on one side, and a small natural ramp on the other, leading to the Central Powers trench.
The map was made by Wickit, and the earliest screenshots of it dates back to 2006. Due to the simplicity of the map, it has become a favourite alongside Somme.
Passchendaele is going to be included in 2.0.