REGILLUS, LAKE OF, situated in the territory of Tusculum in Latium, was celebrated in Roman story as the scene of a great battle. Here it was that the dictator A. Postumius Albus engaged the Latin confederacy under the expelled Tarquins and Mamilius of Tusculum; and here it was that Castor and Pollux appeared on two white steeds, heading the charge of the Roman cavalry against the flying foe. At the present day Regillus has not been identified with certainty; but it is now generally supposed to be the same as the Lago di Cornufelle. That lake, about half a mile in diameter, lying at the foot of the hill on which the modern Frascati is built, is of a singular character. Its basin is evidently the crater of an extinct volcano. Its waters have been completely drained away by an emissary made in the seventeenth century. The dry channel during summer swarms with vipers, so that it is dangerous to approach it. The ruins of an old Roman villa are the only vestige of a building in its immediate neighbourhood.