Take a walk into Lake Nemi as CMC Green, the author of Roman Religion and the Cult of Diana at Aricia describes the setting:
"From here (the narrow road to the crater) the road curves, cresting the crater's ridge, and one finds oneself in a world of incredible remoteness and indeed, sacredness, even while the noise of school children playing has not yet died away. ...
Within the crater the woods are thick and green and birdsong fills the air, and the perfect blue of the lake reflects the sky and the crater's sloping sides. The old (asphalted) Roman road, still slopes gently down from the crest to skirt the lake, moving away toward the sanctuary at a point where the lake leaves a blunt triangle. To pass over the ridge into the crater is a very real experience of crossing a boundary into what is still effectively sacred space.
The crater is steep and the lake 30 meters deep, with only a narrow shoreline. On the northeast side is the one bit of level land beside the lake, roughly triangular in shape. ... Just to the east beneath the sheerest rise of stone, a great spring poured from the crater wall and into the lake. ... The crater is large enough to define a world of its own but not so large that it invites fragmentation of its interior space and in this very contained space, the triangular flatland forms a single orienting point.
The crater is the natural place for a hunting cult. It is the universe writ small, uniting the three cosmic levels - the earth, the canopy of the heavens, and, through the depths of the lake and the caves at its shore, the underworld. It was then, as now, a place of stunning beauty and peace, sacred and remote. ...
It is a small lake, spring fed, ... As there was no above-ground outlet, the lake level would have changed according to seasonal rain or drought although over time such changes would be modertated by the underground outflows. ... This triangle, broad across the curve of the lake and narrowing into the crater walls, is where the terrace for the lower sanctuary was eventually built. It resembled the stubby handle embracing the bottom of an Etruscan mirror. On a calm day the lake reflects the sky and the shoreline with shimmering perfection. The Romans, much more attuned to the visual impact of the setting than we can be, called the lake the speclum Dianae.
The crater has its own microclimate. Violent thunderstorms, earthquakes and thick fogs are all more frequent there then they are in the plains of Latium beyond. Trees - today chestnuts but in antiquity also beech and oak - as well as scrub and the larger woody bushes grew thick on the crater slopes: the area was densely populated with wildlife."
Imagine the ancient celebration as described by Ovid, from his Fasti:
In the Arician valley, there is a lake surrounded by shady forests,
Held sacred by a religion from the olden times...
On a long fence hang many pieces of woven thread,
and many tablets are placed there
as grateful gifts to the Goddess.
Often does a woman whose prayers Diana answered,
With a wreath of flowers crowning her head,
Walk from Rome carrying a burning torch...
There a stream flows down gurgling from its rocky bed..."
One Roman poet, Propertius, apparently attended the festival in the 1st century CE, as indicated in these words to his beloved:
"Ah, if you would only walk here in your leisure hours.
But we cannot meet today,
When I see you hurrying in excitement with a burning torch
To the grove of Nemi where you
Bear light in honour of the Goddess Diana"