CM meaning in Military and Defence ?

Tunnel warfare is a general name for war being conducted in tunnels and other underground cavities. It often includes the construction of underground facilities (mining or undermining) in order to attack or defend, and the use of existing natural caves and artificial underground facilities for military purposes. Tunnels can be used to undermine fortifications and slip into enemy territory for a surprise attack, while it can strengthen a defence by creating the possibility of ambush, counterattack and the ability to transfer troops from one portion of the battleground to another unseen and protected. Also, tunnels can serve as shelter for combatants and non-combatants from enemy attack.

Since antiquity, sappers have used mining against a walled city, fortress, castle or other strongly held and fortified military position. Defenders have dug counter-mines to attack miners or destroy a mine threatening their fortifications. Since tunnels are commonplace in urban areas, tunnel warfare is often a feature, though usually a minor one, of urban warfare. A good example of this was seen in the Syrian Civil War in Aleppo, where in March 2015 rebels planted a large amount of explosives under the Syrian Air Force Intelligence Directorate headquarters.

Tunnels are narrow and restrict fields of fire; thus troops in a tunnel usually have only a few areas exposed to fire or sight at any one time. They can be part of an extensive labyrinth and have cul-de-sacs and reduced lighting, typically creating a closed-in night combat environment.