In early Hellenic cosmogony Tartaros was the great pit beneath the earth. The cosmos was imagined as a great sphere or void, with the upper half of its shell formed by the dome of heaven, and the lower half by the pit of Tartaros. Inside, this cosmic sphere was divided in two by the flat disc of earth. Above was the dwelling place of gods and men, and below was the gloomy, storm-wracked prison of the Titanes. Haides, the realm of the dead, was originally quite distinct from the pit of Tartaros. It was located either at the very ends of the earth, beyond the river Okeanos and the setting of the sun; or in the hollow depths of earth's belly. Tartaros on the other hand, lay as far beneath Haides as the sky lay above the earth. Tartaros was secured with a surrounding wall of bronze set with a pair of gates, guarded by the hundred-handed Hekatonkheir giants, warders of the Titanes.

Later classical writers reimagined Tartaros as the hellish prison-house of the damned. Plato, in 'Gorgias' describes the then-modern views on Tartaros:

"Now in the time of Kronos there was a law concerning mankind, and it holds to this very day amongst the gods, that every man who has passed a just and holy life departs after his decease to the Isles of the Blest (Nesoi Makaron), and dwells in all happiness apart from ill; but whoever has lived unjustly and impiously goes to the dungeon of requital and penance which, you know, they call Tartaros." [523a]

Throughout Hellenic mythology, quite a few people were locked away in Tartaros and for no reason at all, I felt the need to collect some of these today, along with their cautionary tales.

King Sísyphos
Sísyphos (Σίσυφος) was a king; the king of Ephyra, the area now known as Corinth to be exact. For those who enjoy figuring out the convolutd family trees of the ancient kings, he was the son of King Aeolus of Thessaly and Enarete, and the father of Glaucus, Ornytion, Almus, and Thersander by the nymph Merope, the brother of Salmoneus, and the grandfather of Bellerophon through Glaucus. He was a very bright man, with a good mind for ruling. Unfortunately, he was also a proud man, and a deceitful one at that. He ruled his city with an iron fist, and killed visitors, breaking xenia and raising the ire of Zeus Xenios.

Sísyphos is not generally known for his deeds in life; he is far better known for his deeds in death. According to Hellenic mythology, Sísyphos is condemned to roll a rock up to the top of a mountain in Tartaros, only to have the rock roll back down to the bottom every time he reaches the top. The Gods, it seems, are well aware that working a dead end job without pay-off is one of the worst things that can happen to a person. There are various stories about Sísyphos and how he earned this fate. Apollodorus, in his Library, writes the following:

"Sisyphus is punished in Hades by rolling a stone with his hands and head in the effort to heave it over the top; but push it as he will, it rebounds backward. This punishment he endures for the sake of Aegina, daughter of Asopus; for when Zeus had secretly carried her off, Sisyphus is said to have betrayed the secret to Asopus, who was looking for her." [1.9.3]

According to Pherecydes (Frag. 78 in Müller, Fragmenta Historicum Graecorum, i. p. 91) Sísyphos told Asopus that Zeus had carried off his daughter Aegina, but continues to say that Zeus punished him by sending Death after him. Cunning as he was, Sísyphos managed to trick Death and bound him, so that men ceased to die, until Ares came to the rescue, released Death, and gave Sísyphos to Him. Before he died, however, Sísyphos told his wife Merope to omit his funeral rites, so that Hades, being deprived of his customary offerings, would be persuadable to let him go back to life in order to complain of his wife’s neglect. Hades did, indeed, let him go to deal with his wife, but when he refused to return, and had to be fetched back by Hermes, his well-known punishment ensued.

King Tántalos
This king cut up his son Pelops, boiled him, and served him as food when he was invited to dine with the Gods. He also stole the ambrosia from the Gods and told his people its secrets. Another story mentioned that he held onto a golden dog forged by Hephaestus and stolen by Tántalos' friend Pandareus. Tantalus (Τάνταλος) held onto the golden dog for safekeeping and later denied to Pandareus that he had it. Tántalos' punishment for his actions was to stand in a pool of water beneath a fruit tree with low branches. Whenever he reached for the fruit, the branches raised his intended meal from his grasp. Whenever he bent down to get a drink, the water receded before he could get any. Over his head towered a threatening stone like that of Sisyphus. From the 'Odysseia':

"I [Odysseus] saw Tantalus in agonising torment, in a pool of water reaching to his chin. He was tortured by thirst, but could not drink, since every time he stooped eagerly the water was swallowed up and vanished, and at his feet only black earth remained, parched by some god. Fruit hung from the boughs of tall leafy trees, pears and pomegranates, juicy apples, sweet figs and ripe olives. But whenever the old man reached towards them to grasp them in his hands, the wind would sweep them off into the shadowy clouds." [Bk XI:541-592]

Ixion (Ἰξίων), king of the Lapiths (Λαπίθαι), fell in love with Hera after being invited up to Olympos by Zeus. Zeus decided to test his integrity after He discovered His guest' lust for Hera. Zeus created the cloud nymph Nephele (Νεφέλη)in Hera's image. Ixion made love to her and fathered the Kentaurs (or just one, Kentaurus, who became the father of the Centaur race). Needless to say, Ixion failed miserably. He was expelled from Olympus and blasted with a thunderbolt. Zeus then ordered Hermes to bind Ixion to a winged fiery wheel that was always spinning. Therefore, Ixion is bound to a burning solar wheel for all eternity, at first spinning across the heavens, then into Tartaros. The Corona Australis is this very wheel, with Ixion tied to it. Apollodorus, in his 'Bibliotheca': 

"Ixion fell in love with Hera and tried to rape her, and when Hera told Zeus about it, Zeus wanted to determine if her report was really true. So he fashioned a Cloud (Nephele) to look like Hera, and laid it by Ixion's side. When Ixion bragged that he had slept with Hera, Zeus punished him by tying him to a wheel, on which he was turned by winds up in the air. The Cloud (Nephele) bore Kentauros (Centaurus) from Ixion’s seed." [E.1.20]

The Danaides
It is said that these fifty daughters of Danaus (or just forty-nine, or forty-eight, or forty-seven of them) murdered their husbands and were punished in Tartarus by being forced to carry water in a jug to fill a bath which would thereby wash off their sins, but the jugs were actually sieves so the water always leaked out. Ovid in his 'Metamorphoses' poetically describes their fate:

"The Belides their leaky vessels still
Are ever filling, and yet never fill:
Doom'd to this punishment for blood they shed,
For bridegrooms slaughter'd in the bridal bed."

King Salmoneus
This king may have banked himself a place in Tartaros by trying to pass himself off as Zeus, causing the real Zeus to smite him with a thunderbolt. He impersonated the divinity by driving around in a chariot dragging bronze kettles to make thunder, and casting torches in the air for lightning. Salmoneus was the great-grandson of Deukalion, survivor of the Great Deluge. Virgil, in his 'Aeneid':

"Salmoneus, suff'ring cruel pains, I found, For emulating Jove; the rattling sound Of mimic thunder, and the glitt'ring blaze Of pointed lightnings, and their forky rays. Thro' Elis and the Grecian towns he flew; Th' audacious wretch four fiery coursers drew: He wav'd a torch aloft, and, madly vain, Sought godlike worship from a servile train. Ambitious fool! with horny hoofs to pass O'er hollow arches of resounding brass, To rival thunder in its rapid course, And imitate inimitable force! But he, the King of Heav'n, obscure on high, Bar'd his red arm, and, launching from the sky His writhen bolt, not shaking empty smoke, Down to the deep abyss the flaming felon strook." [6..585]