Release the mouse at this point and the engineers will ask you whether you want to pay for it or not. Tunnel-building is expensive, so it pays to be careful about where you go digging. If you choose not to build the tunnel when they ask you, they�ll go ahead and do their best to make a road up the side of the hill. They'll most likely have to mess around with the terrain to make it flat enough for Sims to drive on. If you say yes, then the tunnel will be built. Remember to make sure that the terrain at the future exit is a slope and not jagged.