Quote Originally Posted by dehro View Post
thing is, the heavy weaponry is made of bending too. if I'm an earthbender and you stick me on an island, I just burrow down, start digging a tunnel, and then walk underground towards the nearest coast. you'll never even know I left the island.
setting aside that earth bending jailers is an option. all the armies are primarily composed of non-benders. catapults, ballista, and other high power weaponry is available.

preventing tunneling is just a matter of proper surveying. it's just not possible to tunnel through certain ground.