The Flood story recounted to Gilgamesh by Utnapishtim is markedly similar to the account of the flood in the Bible. However, an Akkadian epic dating back to the 18th Century BCE features a nearly identical flood story involving a character named Atrahasis. Although the Epic of Atrahasis is fragmentary in much the same way the Gilgamesh epic is, there are three tablets which comprise the most complete version of the story.
Atrahasis was a Sumerian King of Shuruppak before the flood. The first tablet of the epic is a creation story, explaining the origin of humankind. The goddess Mami creates humanity out of clay to do the work that lesser gods refuse to do. Tablet II deals with overpopulation, as Enlil uses alternating periods of drought and famine to reduce the population and keep it under control. Enlil eventually decides to destroy humanity with a flood.
Tablet III features an account of the flood, which was likely adapted for the Epic of Gilgamesh. In it, Enki, the god of water, warns Atrahasis of the coming flood. He does this in exactly the same manner that Ea warns Utnapishtim, by speaking to him through the reed walls of his house. He instructs Atrahasis to tear down his home and build a boat. Enki tells Atrahasis that the boat will need multiple decks and must be of great size.
When the boat is complete, Atrahasis boards it with his family and animals. The storm begins and then rages for seven days. When it ends, Atrahasis goes ashore and makes a sacrifice to the gods, again, just as Utnapishtim did. In this version, Enlil is angry with Enki for sharing the secret of the flood with Atrahasis. Enki argues that he has done what was necessary to preserve life. The two gods agree that they must develop another method to control the population, but in this version, they do have a reason for bringing the flood. Utnapishtim's account offers no explanation from the gods for the extermination of humanity.
The similarities are obvious when one compares the contents of Tablet III to that of Tablet XI in Gilgamesh's story. However, Atrahasis's story has no mention of a reward from the gods. This may be because Enki takes the lion's share of the credit for saving humanity, whereas Ea denied telling Utnapishtim anything directly. Regardless, Atrahasis is not granted immortality in the most complete version of the epic.
Many scholars believe this story was directly adapted for use in the Epic of Gilgamesh, arguing that the names Atrahasis and Utnapishtim may have even referred to the same individual. Ea addresses Utnapishtim as the "son of Ubartutu," and Atrahasis is also said to be the son of Ubartutu. This strengthens the argument that the flood story in Gilgamesh comes directly from the Epic of Atrahasis, but there is still debate among scholars.