§ Of the Sindar The Silmarillion

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.