Not the least interesting among the experiences of a sojourn at Bitlis will be the excursion to the so-called tunnel of Semiramis. You follow the course of the river for a distance of some four miles below the castle along the avenue of communication with Sert. A metalled road has been constructed for some portion of the way, representing the abortive attempts to connect the two centres by a carriageable chaussée. It breaks off within 1 1/2 miles of the tunnel, to be succeeded by sporadic patches of levelled inclines. These fitful reminders of the puny civilisation of the present day struggle forward for no great space into the alpine scene. Limestones on the heights above, dark lavas in the trough below accompany your course. Mineral springs well up in abundance along the path. The tunnel is an artificial work, attributed to the Assyrian queen, which pierces a wall of rock blocking the narrow valley and completely cutting off the path (Fig. 148). The barrier has been formed by deposits of lime and other ingredients left by a spring bubbling in a basin some 150 feet above the track and over 300 feet above the right bank of the river. The water in the pool is clear as crystal to the eye, but it tastes strongly of iron. Iron rust reddens portions of the surface of the rock, and is conspicuous on the huge boulders in the bed of the river, detached by the hissing torrent from the base of the parapet. The tunnel has a depth of 22 feet and a height of about 18 feet. It seemed to constitute the only egress from the gorge. The view from this standpoint, looking down the passage of the mountains, is in the sternest vein of alpine landscapes (Fig. 149).