Subterranean Battles are battles that takes place beneath the Earth's surface, in caves, caverns, tunnels, catacombs, or hollow parts of the earth. This may include battles in caverns of mountains, etc.. This is a type of battle that can work well for Ambush battles as (depending on enemy) if they are not expecting underground attacks would be caught off guard.
Though if a surface army tries to invade an army that exists underground, they would face several obstacles such as environmental hazards, toxins, temperature flux, lack of oxygen, darkness, etc..
The battlefield can be anywhere in the world as long as it is terrestrial, But it would help if there is a notable location that is subterranean.
How far apart will both armies be when they begin to fight? How low below the surface are they? What will the their size be? What are their powers, skills, armaments, strategies and tactics be? who will lead them?