Evaporation was given by Aristotle as a reason why the sea does not overflow its bed on account of the constant inflow from the rivers.[118] Another explanation of this puzzling circumstance was found by Pliny[119] in a curious theory that prevailed throughout antiquity and the Middle Ages to the effect that the land is seamed with veins, cavities, and tunnels.[120] Into some of these the air enters; others are the passages for rivers which sink into the ground; through still others the water of the sea finds its way to wells, springs, and fountains, where, made fresh by its passage through the earth, it bursts forth to form rivers which return it to the sea. A continuous circulation of the waters of the earth is thus maintained through passages corresponding to the veins, arteries, and canals of the human body.[121] The origin of the latter theory is undoubtedly to be sought for partly in the nature of the ground in Greece and the Aegean region and partly in the age-old belief that the interior of the earth is the abode of the dead. The soluble character of the limestone rocks throughout parts of the Balkan Peninsula has led to the production of what is now known as _karst_ topography, so called from the Karst, a plateau between Trieste and Fiume, where it has attained its most typical development. In such regions many streams disappear into hollows of the ground; caverns and underground galleries are extremely common; and the traveler occasionally comes across a full-grown river bursting out of the depths of the earth. The old and persistent story that the river Alpheus of the Peloponnesus passes beneath the Ionian Sea only to gush forth in the well of Arethusa in Syracuse was destined to have a medieval counterpart in the explanation of the subterranean courses of the rivers of Paradise. RIVERS OF THE UNDERWORLD