Tunnel warfare is a general name for war being conducted in tunnels and other underground cavities. It often includes the construction of underground facilities in order to attack or for defence and the use of existing natural caves and artificial underground facilities (catacombs, city communications, etc.) for military purposes. Tunnels can be used to undermine fortifications and slip into territory, 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.