Lake Nemi is an ancient lake that is located east of Rome in the Alban Hills where the temple of Diana once stood, surrounded by her sacred grove. In ancient times the lake was known as «Diana’s Mirror» because the reflection of the moon upon Lake Nemi could be perfectly viewed from the temple.
A stream flowed into Lake Nemi from a sacred grotto nearby the temple. This stream was associated with the water nymph named Egeria. Both Egeria and Diana are early forms in the Lady of the Lake mythos. The Lady of the Lake was intimately connected with the guardian of the sacred grove of Diana. He was referred to as Rex Nemorensis, the King of the Woods, whose role it was to guard Diana’s sanctuary at Nemi.
It might be mentioned that the Irish word neimed that means «sanctuary» is uniquely similar to the word «Nemi» itself, which comes from the Latin nemus, meaning «sacred grove.» A.G.H.
Source: 78, 213-214.