The Silmarillion

It came to pass during the second age of the captivity of Melkor that Dwarves came over the Blue Mountains of Ered Luin into Beleriand. Themselves they named Khazâd, but the Sindar called them Naugrim, the Stunted People, and Gonnhirrim, Masters of Stone. Far to the east were the most ancient dwellings of the Naugrim, but they had delved for themselves great halls and mansions, after the manner of their kind, in the eastern side of Ered Luin; and those cities were named in their own tongue Gabilgathol and Tumunzahar. To the north of the great height of Mount Dolmed was Gabilgathol, which the Elves interpreted in their tongue Belegost, that is Mickleburg; and southward was delved Tumunzahar, by the Elves named Nogrod, the Hollowbold. Greatest of all the mansions of the Dwarves was Khazâd-dûm, the Dwarrowdelf, Hadhodrond in the Elvish tongue, that was afterwards in the days of its darkness called Moria; but it was far off in the Mountains of Mist beyond the wide leagues of Eriador, and to the Eldar came but as a name and a rumour from the words of the Dwarves of the Blue Mountains.

From Nogrod and Belegost the Naugrim came forth into Beleriand; and the Elves were filled with amazement, for they had believed themselves to be the only living things in Middle-earth that spoke with words or wrought with hands, and that all others were but birds and beasts. But they could understand no word of the tongue of the Naugrim, which to their ears was cumbrous and unlovely; and few ever of the Eldar have achieved the mastery of it But the Dwarves were swift to learn, and indeed were more willing to learn the Elventongue than to teach their own to those of alien race. Few of the Eldar went ever to Nogrod and Belegost, save Eöl of Nan Elmoth and Maeglin his son; but the Dwarves trafficked into Beleriand, and they made a great road that passed under the shoulders of Mount Dolmed and followed the course of the River Ascar, crossing Gelion at Sarn Athrad, the Ford of Stones, where battle after befell. Ever cool was the friendship between the Naugrim and the Eldar, though much profit they had one of the other; but at that time those griefs that lay between them had not yet come to pass, and King Thingol welcomed them. But the Naugrim gave their friendship more readily to the Noldor in after days than to any others of Elves and Men, because of their love and reverence for Aulë; and the gems of the Noldor they praised above all other wealth. In the darkness of Arda already the Dwarves wrought great works, for even from the first days of their Fathers they had marvellous skill with metals and with stone; but in that ancient time iron and copper they loved to work, rather than silver or gold.

Now Melian had much foresight, after the manner of the Maiar; and when the second age of the captivity of Melkor had passed, she counselled Thingol that the Peace of Arda would not last for ever. He took thought therefore how he should make for himself a kingly dwelling, and a place that should be strong, if evil were to awake again in Middle-earth; and he sought aid and counsel of the Dwarves of Belegost They gave it willingly, for they were unwearied in those days and eager for new works; and though the Dwarves ever demanded a price for all that they did, whether with delight or with toil, at this time they held themselves paid. For Melian taught them much that they were eager to learn, and Thingol rewarded them with many fair pearls. These Círdan gave to him, for they were got in great number in the shallow waters about the Isle of Balar; but the Naugrim had not before seen their like, and they held them dear. One there was as great as a dove's egg, and its sheen was as starlight on the foam of the sea; Nimphelos it was named, and the chieftain of the Dwarves of Belegost prized it above a mountain of wealth.

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.