Hesiod's Tartarus

For a brazen anvil falling down from heaven nine nights and days would reach the earth upon the tenth: and again, a brazen anvil falling from earth nine nights and days would reach Tartarus upon the tenth. Round it runs a fence of bronze, and night spreads in triple line all about it like a neck-circlet, while above grow the roots of the earth and unfruitful sea. There by the counsel of Zeus who drives the clouds the Titan gods are hidden under misty gloom, in a dank place where are the ends of the huge earth. And they may not go out; for Poseidon fixed gates of bronze upon it, and a wall runs all round it on every side. There Gyes and Cottus and great-souled Briareus live, trusty warders of Zeus who holds the aegis.

And there, all in their order, are the sources and ends of gloomy earth and misty Tartarus and the unfruitful sea and starry heaven, loathsome and dank, which even the gods abhor.

It is a great gulf, and if once a man were within the gates, he would not reach the floor until a whole year had reached its end, but cruel blast upon blast would carry him this way and that. And this marvel is awful even to the deathless gods.

There stands the awful home of murky Night wrapped in dark clouds. In front of it the son of Iapetus [Atlas] stands immovably upholding the wide heaven upon his head and unwearying hands, where Night and Day draw near and greet one another as they pass the great threshold of bronze: and while the one is about to go down into the house, the other comes out at the door.

And the house never holds them both within; but always one is without the house passing over the earth, while the other stays at home and waits until the time for her journeying come; and the one holds all-seeing light for them on earth, but the other holds in her arms Sleep, the brother of Death, even evil Night, wrapped in a vaporous cloud.

And there the children of dark Night have their dwellings, Sleep and Death, awful gods. The glowing Sun never looks upon them with his beams, neither as he goes up into heaven, nor as he comes down from heaven. And the former of them roams peacefully over the earth and the sea's broad back and is kindly to men; but the other has a heart of iron, and his spirit within him is pitiless as bronze: whomsoever of men he has once seized he holds fast: and he is hateful even to the deathless gods.
(Hesiod, Theogony, 715ff)