The primordial ocean of Babylonian mythology: mother of the gods, then their enemy, slain by Marduk and split in half to become the sky above and the waters below. Creation, built from a corpse.
Documented
The Enuma Elish, tablet by tablet
Documented: seven tablets, copied in the library of Ashurbanipal (7th century BC) from older compositions, recited at the Babylonian New Year. Tiamat and Apsu, salt and fresh water, breed the gods; the gods kill Apsu; Tiamat raises an army of monsters; Marduk kills her with wind and arrow, splits her body, and builds heaven and earth from the halves. The Hebrew tehom, the deep of Genesis 1:2, is the documented cognate of her name.
Assessment: Fully documented text, documented linguistic kinship with Genesis. The direction of influence, and how much of it, is the argument that fills libraries.
Stories
The dragon-fight template
The folk reading files Tiamat as the grandmother of every chaos-dragon: Leviathan, Typhon, Jormungandr, Vritra, the storm god's eternal reptile. Scholars call the pattern chaoskampf, the fight against chaos, and it is documented across the ancient Near East and Indo-European myth. Tiamat is its oldest full text.
Assessment: Documented as comparative pattern, genuinely one of the oldest stories we have in full. The folk flattening is also real: Tiamat is not a dragon in the tablets, the word covers a cosmic sea, and the dragon costume was fitted later.
Interesting claims
Sitchin's planet
The interesting claim, famous in paperback: Zecharia Sitchin read the Enuma Elish as astronomy, Tiamat a destroyed planet, her split halves the Earth and the asteroid belt, the epic a memory of cosmic collision. Millions of copies later, it is the version more people know than the tablets.
Assessment: Documented as publication history, and documented as mistranslation-driven. Assyriologists, including those Sitchin claimed to read, reject the planetary reading wholesale: the text's own geography is mythic, not orbital. File beside Velikovsky, same method, different planet.
Sources and further reading
Enuma Elish, seven tablets, Nineveh copies (7th century BC) of older compositions
Dalley, Myths from Mesopotamia (1989), the standard translation
Lambert, Babylonian Creation Myths (2013), the critical edition
Genesis 1:2, tehom, the documented cognate
Sitchin, The 12th Planet (1976), cited as the claim, not the evidence
Worth asking
If the world is made from a slain monster, what does that make order, victory, or truce?