It is not an uncommon experience for a submarine tunnel to start out in firm soil and unexpectedly to find that this material becomes soft and treacherous as the work proceeds, or that it is intersected by strata of soft material. The method of dealing with this condition will vary with the circumstances, but generally if any considerable amount of soft material has to be penetrated, or if the inflow of water is very large, the firm-ground system of work is changed to one of the methods employed for excavating soft-ground submarine tunnels. The Milwaukee water supply tunnel, described elsewhere, is a notable example of submarine tunnels, began in firm material which unexpectedly developed a treacherous character after the work had proceeded some distance. Occasionally the task of building a submarine tunnel in the river bed arises. In such cases the tunnel is usually built by means of cofferdams in shallow water, and by means of caissons in deep water. Submarine tunnels under rivers are usually built with a descending grade from each end which terminates in a level middle position, the longitudinal profile of the tunnel corresponding to the transverse profile of the river bottom. Where, however, such tunnels pass under the water with one end submerged, and the other end rising to land like the water supply tunnels of Chicago, Milwaukee, and Cleveland, the longitudinal profile is commonly level, or else descends from the shore to a level position reaching out under the water. The drainage of submarine tunnels during construction is one of the most serious problems with which the engineer has to deal in such works. This arises from the fact that, since the entrances of the tunnel are higher than the other parts, all of the seepage water remains in the tunnel unless pumped out, and from the possibility of encountering faults or permeable strata, which reach to the stream bed and give access to water in greater or less quantities. Generally, therefore, the excavation is conducted in such a manner that the inflowing water is led directly to sumps. To drain these sumps pumping stations are necessary at the shore shafts, and they should have ample capacity to handle the ordinary amount of seepage, and enough surplus capacity to meet probable increases in the inflow. For extraordinary emergencies this plant may have to be greatly enlarged, but it is not usual to provide for these at the outset unless their likelihood is obvious from the start. The character and size of the pumping plants used in constructing a number of well-known tunnels are described in Chapter XII.