I suggest that you either construct the various terrain sections with holes in a 3D package instead, OR cleverly design the mine shaft objects such that you can remove a tile (two triangles) from the terrain, and replace it with a tile that has the mine shaft connected to it. This would be FAR FAR easier than general boolean operations. And as a bonus, when the user is far from the mine shaft, you can block the hole in the terrain with a black square so that you don't have to actually draw all the polygons inside the mine until the player gets very close to it. You could even fade the alpha tile out as the player approaches to make the transition smoother, just in case they can actually see the hole as they approach.