Hesiod measures it. A bronze anvil falling from heaven would take nine days and nights to reach the earth on the tenth; an anvil falling from the earth would take nine days and nights to reach Tartarus on the tenth. That is the whole cosmology in one image: three levels, evenly spaced, with the pit as far below the ground as the sky is above it.
Around it runs a bronze wall, and night is poured about the neck of it in three rows, and the roots of the earth and the unfruitful sea grow above. There is a gate of bronze that Poseidon fitted, and beyond it the Titans are shut away, with the three Hundred-Handers — Kottos, Gyges and Briareos — stationed as trusted guards. It is described as a place where a wind would blow a man about for a whole year without letting him touch the ground, and where even the gods are frightened.
A place and a person at once
Hesiod lists Tartarus among the first entities to come into being, alongside Chaos, Gaia and Eros — so it is not a location that was later constructed but one of the original conditions of the world. It then behaves as a person exactly once: Gaia, in her final act of opposition to Zeus, bears Typhon to Tartarus, the hundred-headed monster who nearly overthrows the Olympian order.
That dual nature — a region that is also a parent — is shared with Gaia and, more ambiguously, with Chaos. Greek cosmogony did not distinguish sharply between the world's parts and the world's first beings, and forcing the distinction produces confusion that the sources do not have.
Who is actually in it
The population changes over time in a way that is worth tracking, because it amounts to a theological shift.
In Homer and Hesiod the prisoners are divine. The Titans are the point of the place. Homer has Zeus threaten to throw any disobedient god into "misty Tartarus, very far away, where the deepest gulf beneath the earth is", and adds that it is as far below Hades as heaven is above the earth. It is a maximum-security wing for gods, not a hell for people.
By the classical period it has acquired human inmates, and specifically the great sinners:
- Tantalus, standing in water that recedes when he stoops and under fruit that lifts away when he reaches — the origin of tantalise. His crime, in the usual version, was serving his son Pelops to the gods at a banquet to test them.
- Sisyphus, rolling the stone up and watching it roll down, punished for his deceptions, including chaining Death.
- Ixion, bound to a burning wheel that turns forever, for attempting Hera and for the first kin-murder among mortals.
- Tityos, stretched over nine acres while vultures eat his liver, for assaulting Leto.
- The Danaids, forty-nine sisters carrying water in leaking jars, for killing their husbands on their wedding night.
Homer's Odyssey places Tityos, Tantalus and Sisyphus in the underworld without naming Tartarus, which is one of the seams in the tradition.
By Plato's time it is a place of judgement. The myth at the end of the Gorgias has souls judged and the incurable sent to Tartarus as an example to others, while the curable are punished and released. The Phaedo and the Republic elaborate. That is a fully moralised afterlife with a mechanism of desert, and it is a considerable distance from a bronze-walled cell for defeated Titans.
Virgil's Aeneid 6 completes the picture that medieval Europe inherited: a triple wall, the burning river Phlegethon, a great gate with adamantine columns, Tisiphone on watch with a whip, a hydra with fifty black gaping throats within, and the sound of scourging, dragging chains and groans that Aeneas is not permitted to enter and see.
Tartarus is not Hades, and the distinction matters
Tartarus is not Hades. Hades is the kingdom of the dead in general, where most people go and where the asphodel meadow holds the ordinary shades. Tartarus is a specific deep place within or beneath it, and originally not connected to human death at all. Greek eschatology in its fuller form is roughly tripartite: Elysium or the Isles of the Blessed for the favoured, the asphodel meadow for the mass of the dead, Tartarus for the punished.
That structure — reward, neutrality, punishment — is one of the ancestors of later Western pictures of the afterlife, though the transmission runs through Plato and Virgil and Christian writers rather than directly.
The New Testament uses a verb built on the name once, in 2 Peter, of angels being cast down and kept for judgement, which is the one place the Greek word crossed into Christian scripture and helped fix the association between the classical pit and the Christian hell.
The modern pit
It has become a stock location. Percy Jackson's characters fall into it; games from God of War to Hades use it as a level; fantasy novels use it as a synonym for the deepest possible prison. Almost all of these use the moralised Virgilian version — fire, torment, monsters — rather than Hesiod's, which is cold, windy, bronze-walled and mostly empty except for its prisoners and the guards.
The word survives in English chiefly in tartar — the deposit on teeth and the crust in wine vessels — which comes by a different route through medieval Latin and Arabic, and in the phrase "to catch a Tartar", which is unrelated and refers to Central Asian horsemen. The overlap of Tartarus and Tatar is a coincidence that medieval European writers exploited enthusiastically, calling the Mongol invaders Tartars as if they had come out of hell.
Releases, entrances, and the difference from Erebus
Was anyone ever released?
Yes, and it matters for the mythology's politics. Hesiod has Zeus release the Cyclopes and the Hundred-Handers, whom Ouranos and then Cronus had imprisoned, in order to win the Titanomachy — the Cyclopes give him the thunderbolt in return, and the Hundred-Handers throw the rocks that decide the war. Aeschylus, in Prometheus Bound, also implies the Titans were later released and reconciled, and Pindar places Cronus ruling over the blessed dead. The prison is not necessarily permanent.
Did Greeks believe in it as a real destination?
Most Greeks seem not to have expected personal punishment after death; the Odyssey's picture of a dim, undifferentiated existence in the asphodel meadow is closer to the mainstream view, and the great sinners are exceptional cases suffering for exceptional crimes against the gods. Belief in a judged and sorted afterlife is associated with mystery cults, with Orphic and Pythagorean circles, and with philosophy — that is, with minorities holding a distinctive doctrine.
Where is its entrance?
Several places claimed one. Taenarum in Laconia, a cave at the southern tip of the Mani, was the best known and is where Heracles is usually said to have dragged out Cerberus; others were at Heraclea Pontica, at Lake Avernus in Italy, and at Eleusis. These were real caves and chasms that people could visit, and some had oracles of the dead attached, so the geography of the underworld had physical addresses.
Is Erebus the same thing?
No. Erebus is darkness — a primordial being, child of Chaos, and by extension a name for the gloom of the underworld, sometimes used for the region the dead first pass through. Tartarus is depth rather than darkness, and is specifically penal. Later Roman and post-classical writers use both loosely as synonyms for the underworld, which flattens a real distinction in the Greek.