Travelers who have visited the Bay of Biscay, on the French and Spanish shore near Biarritz, have observed how the rocks have been worn into caverns, arches, alcoves, and honeycombed formations by the action of the waters for centuries. Just so the soft limestone strata beneath the surface of Cuba, in many portions of the island, have been hollowed out, tunneled, and formed into caves, by the tremendous downpour and wash of tropical rains. So the action of the sea has created a cave under Moro Castle, at the mouth of the harbor of Havana, as well as under that other Moro which stands guard over the entrance of Santiago de Cuba. The existence of these subterranean caverns has often led to serious accidents. In some instances buildings which were by chance erected just over them have suddenly been swallowed up as though by an earthquake. Many of the rivers are navigable for short distances. The longest is the Cauto, in the eastern department, which, rising in the Sierra del Cobre, passes between Holguin and Jiguani, and empties on the south coast a little north of Manzanillo. It is navigable for half its length, between fifty and sixty leagues. The river Ay has falls in its course two hundred feet high, and a natural bridge spanning it, nearly as remarkable as that of Virginia. The Sagua le Grande is navigable for five leagues, and the same may be said of the river Sasa. The Agabama, emptying on the south coast near Trinidad, is also partially navigable. There are two hundred and sixty rivers in all, independent of rivulets and torrents. So abundantly is the island supplied with fresh-water springs, especially on the south side, that the pure liquid filters through the fissures of the stratified rock in such quantities as to form, by hydrostatic pressure, springs in the sea itself some distance from the shore. The sulphurous and thermal springs of San Diego are the resort of numerous invalids annually, who come hither from Europe and America.