Regillus was an ancient lake of Latium

Latium

Lazio is one of the 20 administrative regions of Italy, situated in the central peninsular section of the country. With about 5.7 million residents and a GDP of more than 170 billion euros, Lazio is the third most populated and the second richest region of Italy...

, Italy

Italy

Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

, famous in the legendary history of Rome

History of Rome

The history of Rome spans 2,800 years of the existence of a city that grew from a small Italian village in the 9th century BC into the centre of a vast civilisation that dominated the Mediterranean region for centuries. Its political power was eventually replaced by that of peoples of mostly...

 as the lake in the neighborhood of which occurred (496 B.C.) the battle

Battle of Lake Regillus

The Battle of Lake Regillus was a legendary early Roman victory, won over the Latin League led by the expelled Etruscan former king of Rome. It is usually said to have occurred in 498 BC, but other dates have been proposed, including 499 BC, 496 BC and 493 BC.The battle may be entirely legendary,...

 which finally decided the hegemony of Rome in Latium. During the battle, so runs the story, the dictator Postumius

Postumius

Postumius can mean:*Three consuls called Lucius Postumius Albinus:**Lucius Postumius Albinus , ambushed by the Celtic Boii in 216 BC**Lucius Postumius Albinus **Lucius Postumius Albinus...

 vowed a temple to Castor and Pollux

Castor and Pollux

In Greek and Roman mythology, Castor and Pollux or Polydeuces were twin brothers, together known as the Dioscuri . Their mother was Leda, but Castor was the mortal son of Tyndareus, king of Sparta, and Pollux the divine son of Zeus, who visited Leda in the guise of a swan...

, who were specially venerated in Tusculum

Tusculum

Tusculum is a ruined Roman city in the Alban Hills, in the Latium region of Italy.-Location:Tusculum is one of the largest Roman cities in Alban Hills. The ruins of Tusculum are located on Tuscolo hill—more specifically on the northern edge of the outer crater ring of the Alban volcano...

, the chief city of the Latins (it being a Roman usage to invoke the aid of the gods of the enemy), who appeared during the battle, and brought the news of the victory to Rome, watering their horses at the spring of Juturna

Juturna

In the myth and religion of ancient Rome, Juturna was a goddess of fountains, wells and springs. She was a sister of Turnus and supported him against Aeneas by giving him his sword after he dropped it in battle, as well as taking him away from the battle when it seemed he would get killed...

, close to which their temple in the Forum was erected. There can be little doubt that the lake actually existed. Of the various identifications proposed, the best is that of Prof. L. Pareti, who finds it in a now dry crater lake (Prataporci), drained by an emissarium, the date of which is uncertain, north of Frascati

Frascati

Frascati is a town and comune in the province of Rome in the Lazio region of central Italy. It is located south-east of Rome, on the Alban Hills close to the ancient city of Tusculum. Frascati is closely associated with science, being the location of several international scientific...

. Most of the other sites proposed are not, as Regillus should be, within the limits of the territory of Tusculum.