The limestone rocks of the interior of the Peloponnesus abound as much in chasms, which swallow up the rivers, as do Bœotia and the western portion of the whole of the Balkan peninsula. Some of these katavothras are mere sieves, hidden beneath herbage and pebbles, but others are wide chasms and caverns, through which the course of the underground waters may be readily traced. In winter wild birds post themselves at the entrances of these caverns, in expectation of the prey which the river is certain to carry towards them; in summer, after the waters have retired, foxes and jackals again take possession of their accustomed dens. The water swallowed up by these chasms on the plateau reappears on the other side of the mountains in the shape of springs, or _kephalaria_ (_kephalovrysis_). The water of these springs has been purified by its passage through the earth, and its temperature is that of the soil. It bursts forth sometimes from a crevice in the rocks, sometimes in an alluvial plain, and sometimes even from the bottom of the sea. The subterranean geography of Greece is not yet sufficiently known to enable us to trace each of these kephalaria to the katavothras which feed them. [Illustration: Fig. 19.—LAKES PHENEA AND STYMPHALUS. From the French Staff Map. Scale 1 : 500,000.] The ancients were most careful in keeping open these natural funnels, for, by facilitating the passage of the water, they prevented the formation of swamps. These precautions, however, were neglected during the centuries of barbarism which overcame Greece, and the waters were permitted to accumulate in many places at the expense of the salubrity of the country. The plain of Pheneus, or Phonia, a vast chasm between the Aroanian Mountains and the Cyllene, has thus repeatedly been converted into a lake. In the middle of last century the whole of this basin {61} was filled with water to a depth of more than 300 feet. In 1828, when this sheet of water had already become considerably reduced, it was still 6 miles long and 150 feet in depth. At length, a few years afterwards, the subterranean sluices opened, the waters disappeared, and there remained only two small marshes near the places of exit. But in 1850 the lake was again 200 feet in depth. Hercules, we are told, constructed a canal to drain this valley and to cleanse its subterranean outlets, but the inhabitants content themselves now with placing a grating above the “sink-holes,” to prevent the admission of trunks of trees and of other large objects carried along by the floods.