The Door of Night (Silmarillion) III

The Door of Night was likened to a huge gate of stone at the very edge of the world: the Ilurambar, invisible Walls of the World.

"There it still stands, utterly black and huge against the deep-blue walls. Its pillars are of the mightiest basalt and its lintel likewise, but great dragons of black stone are carved thereon, and shadowy smoke pours slowly from their jaws."