Tunnels solved movement and communication problems

Explosive mines could remove a fortified position but leave a crater that obstructed the infantry expected to advance through it. Underground work therefore served several purposes beyond destroying enemy strongpoints. Approach tunnels brought troops close to a trench so they could emerge during the final bombardment. Protected routes also helped reinforce and supply newly captured positions while artillery barred movement across the former no man's land. A captured trench's defences faced the wrong way, and severed telephone lines or blocked runners could leave its defenders isolated before the counterattack arrived.