In 1872 George Smith, a former banknote engraver working as an assistant at the British Museum, was reading through unbaked clay fragments from Nineveh and found an account of a flood: a man told to build a boat, the waters, the vessel grounding on a mountain, birds released to test for dry land, a sacrifice on landing at which the gods gathered.
The passage is the eleventh tablet of the epic, and the flood survivor is Utnapishtim. Its resemblance to Genesis is not general, it is close in structure and in specific details, including the sequence of released birds.
Smith announced the discovery to the Society of Biblical Archaeology in December 1872, and the reaction was enormous, because the tablet was demonstrably older than the biblical text and because the implication was obvious. A newspaper funded him to return to Nineveh to look for the missing lines, which he found. He died of dysentery in Aleppo in 1876, aged thirty-six.
The flood narrative is older than the epic itself. It appears in the Atrahasis poem, an Old Babylonian composition where the gods send the flood because humanity has become too noisy for them to sleep, and there is a Sumerian version with a survivor named Ziusudra. The epic borrowed it.
What the relationship to Genesis is remains argued: direct literary dependence, a common Mesopotamian inheritance transmitted during the exile, or independent responses to real inundations in a flood plain. That there is a relationship is not seriously contested. That Genesis is the earlier of the two is not defensible.