The mountain at the southern end of the plain, opposite Cyllene, was called Apelaurum (τὸ Ἀπέλαυρον), and at its foot is the subterranean outlet of the lake of Stymphalus (ἡ Στυμφαλὶς λίμνη or ἡ Στυμφηλίη λίμνη). This lake is formed partly by the rain-water descending from Cyllene and Apelaurum, and partly by three streams which flow into it from different parts of the plain. From the west descends a small stream, which rises in Mount Geronteium in the neighbourhood of Kastanía; and from the east comes another stream, which rises near Dusa. But the most important of the three streams is the one which rises on the northern side of the plain, from a copious subterranean outlet; this stream was called Stymphalus by the ancients; it was regarded by them as the principal source of the lake, and was universally believed to make its reappearance, after a subterranean course of 200 stadia, as the river Erasinus in Argolis