Why is there a need to go to the surface because a troll is there?

Trolls are the meanest creatures in the deep-tunnels and when they get to the surface, they often destroy everything in their paths. So it is necessary to retrieve them before they do damage to human quarters or hurt a human. The troll is stupid and won't listen to reason, so it must be overcome. But because it is so large and so determined to tunnel, it must be handled with caution. If the troll is not stopped in its destructive path, the humans might see it thus exposing the People who live underground.