Explain why there were multiple layers to the trench system?

Long, narrow trenches dug into the ground at the front, usually by the infantry soldiers who would occupy them for weeks at a time, were designed to protect World War I troops from machine-gun fire and artillery attack from the air.

The deeper they dug, the more water would seep in, whether it be from below them or from the rain. When it became too much for the soldiers, they used boards and built stands.