The similarity between the mountains of Bosnia and of the Jura is rendered complete by the existence of grottoes, sink-holes, and subterranean rivers. Sink-holes from 60 to 100 feet in diameter, and shaped like funnels, are met with in many localities. Several rivers appear suddenly at the foot of a hill, and, after flowing on for a few miles, disappear again beneath some portal in the rocks. The table-land of the Herzegovina especially abounds in phenomena of this kind. The ground there is pierced by “sinks,” or _ponors_, which swallow up the water derived from precipitation. “Blind valleys” and “troughs” present everywhere the traces of currents of water and of temporary lakes, and after heavy rains the subterranean basins sometimes rise to the surface, and a river then flows for a time along the valley. As a rule, however, the inhabitants are compelled to {128} collect the water they require in cisterns, or to fetch it from long distances. Elsewhere the hydrography of the country is subject to annual changes. Lakes which still figure upon our maps are drained through subterranean passages only recently opened; other lakes are formed in consequence of some passage, which formerly carried off the surface water, having become choked with alluvium. No more curious river probably exists in the world than the Trebinishtitza, in the Western Herzegovina. It appears and disappears many times. One of its branches, flowing at one time on the surface, at others underground, crosses the plains of Kotesi, in turns a parched champaign country or a lake abounding in fish, and enters the Narenta. Other branches pass beneath the mountains, and gush out near the shores of the Adriatic. One of the most famous of these springs is that of Ombra, which pours its waters into the Bay of Gravosa, to the north of Ragusa. [Illustration: Fig. 37.—SUBTERRANEAN BEDS OF THE AFFLUENTS OF THE NARENTA. Scale 1 : 1,925,000.]