"Tunnelling", the large, unwanted holes inside the cake, is almost always caused by over-mixing the batter, or using too much leavening. When you mix too much, you develop strong gluten strands (like in bread) that trap big gas bubbles instead of tiny ones. As the cake bakes, these big bubbles force their way up through the batter, leaving behind long, empty tunnels throughout the cake instead of a soft, even crumb.