Therefore the Naugrim laboured long and gladly for Thingol, and devised for him mansions after the fashion of their people, delved deep in the earth. Where the Esgalduin flowed down, and parted Neldoreth from Region, there rose in the midst of the forest a rocky hill, and the river ran at its feet. There they made the gates of the hall of Thingol, and they built a bridge of stone over the river, by which alone the gates could be entered. Beyond the gates wide passages ran down to high halls and chambers far below that were hewn in the living stone, so many and so great that that dwelling was named Menegroth, the Thousand Caves.
But the Elves also had part in that labour, and Elves and Dwarves together, each with their own skill, there wrought out the visions of Melian, images of the wonder and beauty of Valinor beyond the Sea. The pillars of Menegroth were hewn in the likeness of the beeches of Oromë, stock, bough, and leaf, and they were lit with lanterns of gold. The nightingales sang there as in the gardens of Lórien; and there were fountains of silver, and basins of marble, and floors of many-coloured stones. Carven figures of beasts and birds there ran upon the walls, or climbed upon the pillars, or peered among the branches entwined with many flowers. And as the years passed Melian and her maidens filled the halls with woven hangings wherein could be read the deeds of the Valar, and many things that had befallen in Arda since its beginning, and shadows of things that were yet to be. That was the fairest dwelling of any king that has ever been east of the Sea.

(Silm, Chapter 10, Of the Sindar)

Downward with gentle hand she led
through corridors of carven dread
whose turns were lit by lanterns hung
or flames from torches that were flung
on dragons hewn in the cold stone
with jewelled eyes and teeth of bone.
Then sudden, deep beneath the earth
the silences with silver mirth
were shaken and the rocks were ringing
the birds of Melian were singing;
and wide the ways of shadow spread
as into arched halls she led
Beren in wonder. There a light
like day immortal and like night
of stars unclouded, shone and gleamed.
A vault of topless trees it seemed,
whose trunks of carven stone were stood
like towers of an enchanted wood
in magic fast for ever bound,
bearing a roof whose branches wound
in endless tracery of green
lit by some leaf-emprisoned sheen
of moon and sun, and wrought of gems,
and each leaf hung on golden stems.

Lo! there amid immortal flowers
the nightingales in shining bowers
sang o'er the head of Melian,
while water for ever dripped and ran
from fountains in the rocky floor.
There Thingol sat. His crown he wore
of green and silver, and round his chair
a host in gleaming armour fair.

(HoME III, The Lays of Beleriand, The Lay of Leithian, 980-1011)