Lake Nemi (Italian: Lago di Nemi, Latin: Nemorensis Lacus, also called Diana's Mirror, Latin: Speculum Dianae) is a small circular volcanic lake in the Alban Hills 30 km (19 mi) south of Rome in the Lazio region of Italy.

From the 6th century BC, the lake and its forest were sacred to the goddess Diana Nemorensis and the site of the festival Nemoralia.

In Roman times up to 6 large villas lay on the rim of the crater taking advantage of the cool summer air and fine views over the lake and the sea.

Emperors Tiberius and Caligula sailed Lake Nemi not merely to cool off in summer, but to assert themselves as Nemorenses, rulers aligning with the Stars, wedded to Earth's perpetual life-force.

[4] It was built in the late Republican era, mid-first century BC, and underwent 4 remodelling phases until the 120s AD.

The rooms had geometric monochrome floors and a small bath suite north of the villa built in the fourth phase probably also for servants.

Surrounding the peristyle on western and southern sides were living rooms (cubicula) decorated with opus sectile floors and walls with coloured stucco.

Hence the lake level was controlled from rising too high in ancient times by an emissary connected to a 1650 m-long tunnel dug through the crater wall and ending in Vallericca c. 10 m lower.

It dates from before circa 300 BC when the upper inlet was closed and replaced by an entrance around 11 m lower to further reduce the level of the lake and which had a series of chambers.