Harry Thurston Peck, Harpers Dictionary of Classical Antiquities (1898)

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Copāïs

（Κωπαΐς λίμνη). Alake in Boeotia, formed chiefly by the river Cephissus, whose waters were connected with the Euboean Sea by several subterranean channels, called by the modern Greeks katavóthra, which were not, however, sufficient to carry off the waters, especially in the spring when the Copaic plain was flooded by the rains. In the time of Alexander the Great an enormous tunnel was cut through the rock for the discharge of the water. (See Emissarium.) This proved effective until it fell into ruins, when the district again became unwholesome and marshy. In 1886, however, it was once more properly drained by a French company. The modern name of the lake is Topolias; its Homeric name, Cephisis (Λίμνη Κηφισίς, Il. v. 709). Its eels were much prized in antiquity.