Next to a drift, the tunnel is the simplest and most economical method, under certain circumstances, of making an entrance into a mine. This is a passage driven across the measures, and at right angles to the seam, in order to reach coal which at the point of opening is not exposed. The tunnel is usually driven into the side of a hill. The earth is first dug away until the rock is exposed, or, if the soil be too deep for that, only enough of it is taken to make a vertical face for the mouth of the tunnel. The opening is then driven into the hill at about the same width and height that a drift would be made, and in practically the same manner. If there is a section of earth tunneling at the mouth, the timbering must be close, and the lagging will be of heavy planks. When the solid rock is reached, however, it is not often that any timbering is necessary, the sides and roof being so hard and firm as not to need support. This passage is driven against the face of a coal seam, and when the coal is finally reached the tunnel proper ends, a passage is opened to the right and one to the left along the strike of the seam, and from these gangways the coal is mined. The tunnel, like the drift, must be above water level, and its floor must have a descending grade toward the mouth, to carry off water. The expense of the tunnel, and its superiority to the slope or shaft, will depend upon the distance through which the rock must be pierced before coal is reached. It is especially advisable, therefore, before opening a tunnel, to have an accurate map of the location and dip of the coal seams to be struck by it, otherwise no approximate calculation can be made of the extent or cost of the work. In the anthracite districts, where the seams are sharply pitching, tunnels are driven in the interior of a mine from the workings of a seam already opened across the intervening measures to strike an adjacent seam. In this way two, three, or more coal seams can be worked, and the coal can all be brought out at one surface opening. This is virtually the only kind of tunneling that is now done in the anthracite regions; for, as has already been explained, the coal that lay above water level and was thus accessible by tunnel has now been mostly mined out.