Ceto was born in the earliest days of the cosmos, before the Olympian gods had won their throne, before Zeus had hurled his first thunderbolt, before there were mortals to fear the sea. Her mother was Gaia, the earth, who had already given birth to the sky, the mountains, and the sea itself. Her father was Pontus, the sea, who had sprung directly from Gaia without need of a mate, a primordial being of salt and depth. Ceto had four siblings, all of them powers of the ocean: Nereus, the old man of the sea who was gentle and wise, the father of the fifty Nereids, the beautiful sea-nymphs who attended Poseidon; Thaumas, the god of the wonders of the sea, whose own children included Iris, the rainbow messenger of the gods, and the Harpies, the filthy winged creatures with women's faces who snatched food and cursed travellers; Eurybia, the goddess who commanded the external forces of the sea, the winds and the constellations that guided ships; and Phorcys, the old man of the sea who represented its hidden dangers, a creature of the depths whose form was half man and half fish, with crab-claw forelegs and red spiked skin.

The Greeks did not tell stories of Ceto walking upon the land or interfering in the affairs of cities. She belonged to the deep, to the places where light does not reach and where the pressure would crush a mortal man. Her domain was the abyssal plain, the trenches and caves of the ocean floor where no sailor had ever dropped anchor. But though she never left the water, her influence reached every shore, for she and Phorcys produced children who would haunt the Greek imagination for millennia.

Their first children were the Graeae, three sisters born as old women with grey hair and withered faces, personifications of the white foam where the sea meets the land. Their names were Pemphredo, Enyo, and Deino - though some call the third Persis instead of Deino - and they shared a single eye and a single tooth between them, passing the eye from hand to hand and the tooth from mouth to mouth as they needed them. They dwelled in the far west, beyond the river Oceanus that encircles the world, and they knew the secrets of the earth and the underworld. When the hero Perseus, the demigod son of Zeus and Danae, came seeking the location of the Gorgon Medusa, he found the Graeae and stole their shared eye, holding it ransom until they told him what he needed to know.

Next came the Gorgons, three sisters whose faces were so terrible that one look turned any living creature to stone. Their names were Sthenno, Euryale, and Medusa, and they had serpents for hair, tusks like boars, and hands of bronze. Sthenno and Euryale were immortal, but Medusa was mortal, and it was her head that Perseus cut off with a sickle of adamant given to him by the goddess Athena, using her reflected image in his shield to avoid her petrifying gaze. From the blood of the severed head sprang Pegasus, the winged horse, and Chrysaor, the warrior with the golden sword. The Gorgons' heads appeared on the shields of Greek warriors for centuries, a protective image drawn from the terror that Ceto had birthed.

But Ceto's most terrible child was yet to come. In a hollow cave, she bore Echidna, a creature half beautiful nymph and half monstrous serpent, divine and haughty, who made her home in a cavern beneath the earth and preyed upon any traveller who passed too close. Echidna took Typhon, the giant storm god with a hundred serpent heads for fingers, as her mate, and from that union came the monsters that would test the greatest heroes of Greece: the Chimera, a fire-breathing beast with the head of a lion, the body of a goat, and the tail of a serpent; Cerberus, the three-headed hound who guarded the entrance to the underworld; the Lernaean Hydra, the many-headed serpent whose heads grew back two for every one that was cut off; the Nemean lion, whose hide was impervious to weapons; and the Sphinx, the winged creature with a woman's face and a lion's body who devoured those who could not answer her riddle. Every labour of Heracles, the greatest hero of Greece, and every trial of Perseus and Bellerophon, led back eventually to Ceto.

Her last child with Phorcys was a serpent, unnamed in the earliest account by Hesiod but later called Ladon, the dragon who guarded the golden apples in the garden of the Hesperides, the nymphs who tended the tree at the far western edge of the world. Ladon had a hundred heads and never slept, coiled around the trunk of the sacred tree, and it was he who made the apples impossible to steal until Heracles, in his eleventh labour, shot him with arrows poisoned with the blood of the Hydra, another of Ceto's descendants.

Ceto had another name, Crataeis, which means the mighty one or she of the rocks, and under this name she was identified as the mother of Scylla, the six-headed monster who lived in a cave on the Italian side of the strait between Sicily and the mainland, directly across from Charybdis, the whirlpool that swallowed ships whole. When Odysseus, the hero of the Trojan War whose cunning was legendary, sailed his ship through that narrow channel, he lost six of his men to Scylla's heads rather than risk the whole vessel in the whirlpool, and the memory of that passage still haunts the place. Some sources conflated Crataeis with Hecate, the goddess of witchcraft and ghosts who walks the earth by night, suggesting that Ceto's power extended beyond the sea into the realm of magic and darkness.

The Roman naturalist Pliny the Elder, writing in the first century CE, recorded that the people of Joppa on the coast of the Levant worshipped a goddess they called Ceto, the sea monster, at a shrine near the very rock where Andromeda had been chained. Whether this was a genuine cult of the primordial goddess or a local tradition that had attached her name to the monster of that story, the connection placed Ceto at the threshold between the divine and the monstrous, the mother whose children were the terrors that heroes had to overcome.