The difficulties connected with crossing the upper Indus at Attock, a thousand miles from its mouth, and 940 feet above sea level, have long been felt. Colonel Robertson, of the Madras Staff Corps, therefore, proposed a scheme for going under instead of over it. In his report on the subject submitted to Government in 1859, he stated that, as the geological formation at Attock, is a compact slate rock, it is easily worked; and under the bed of the river it is apparently not broken by any great fissures which might possibly endanger the tunnel. Colonel Robertson fixed the upper level of the excavation for the tunnel at 60 feet under low water cold weather mark, or, at the water’s deepest point, 20 feet below the bed of the river; the lining of the tunnel to be 2 feet thick, and as its height is 20 feet, the foundation level would be 82 feet below the low water level. To guard against all risk of inundation through floods, which raise the level of the river from 50 to 92 feet (it was at the latter height in 1841), the two entrances of the tunnel are to be 100 feet above low water level. The width of the river at the point selected is 1,215 feet. This portion of the tunnel is to be nearly on the level; but the gradient of the approaches to it on each side, each 3,720 long, is to be 1 in 40, or at the rate of 132 feet in the mile; the total length of actual tunnel to be 7,215 feet, as some portion of each approach is to be in open cutting. There are ten shafts, each 600 feet apart, except at the actual river, where they are 1,580 feet apart. In 1860 the works were commenced, and a drift gallery had been nearly carried through, when all operations were suspended; but it is intended that they be resumed in prospect of the railway between Lahore and Pesshawer being constructed. The revived estimate makes the cost of the tunnel £105,000, if the gradient on each side be 1 in 20; but if it be flattened to 1 in 30, the estimate is £143,300.[133] The greatest tunnels that we know of connected with mining (irrespective of galleries for working in mines) are the great drainage galleries at the mines of Clausthal, in the Hartz, 11,377 yards, or 6½ miles long, and in many parts 900 feet below the surface of the superjacent mountain. The second is the _Great Adit_, which drains several of the important mines in the parish of Gwennap, Cornwall. It is from 30 to 60 feet below the surface, and is 30 miles long. There is an adit level of 10,000 yards to the celebrated silver-mines of Norway.