Tartarus is the deepest region of the Greek cosmos, a pit lying as far beneath Hades as the earth lies beneath the sky, Hesiod famously measures the distance by imagining a bronze anvil falling for nine days from heaven to earth, and nine more from earth to Tartarus. It is both a primordial entity, one of the first beings to emerge at the origin of things (and, with Gaia, a parent of the monster Typhon), and a place: the ultimate prison of the cosmos.
Into it Zeus casts the defeated enemies of the Olympian order, the Titans, after their overthrow, sealed behind bronze gates and guarded by the hundred-handed Hecatoncheires, and there too suffer the archetypal sinners whose punishments became proverbial: Tantalus, forever reaching for food and water that recede from him; Sisyphus, endlessly rolling his boulder uphill only to watch it roll down; Ixion bound to a burning wheel; the daughters of Danaus filling a leaking jar without end. Tartarus is thus the Greek imagination's image of ultimate confinement and eternal, fitting punishment, the place where cosmic rebellion and the gravest human crimes are put away below everything else. It supplied later Western thought (and, through the Septuagint's use of the word, some New Testament imagery) with a vocabulary for the lowest hell.