In the Epic of Gilgamesh, Utnapishtim uses bitumen to waterproof his boat.
When Ea warns Utnapishtim of the coming flood, the survivor builds a huge vessel and seals it with bitumen. Bitumen is a naturally occurring petroleum-based material that was widely used in ancient Mesopotamia as a waterproofing substance, sealant, and building material. Its use fits the practical setting of a riverine civilization where boats and mud-brick structures needed protection from water.
The flood account in the Epic of Gilgamesh resembles the Genesis story in several memorable details, including divine warning, a constructed vessel, preservation of living beings, and the release of birds. Similarity does not mean the two texts use identical wording or characters. In Genesis, the ark is coated with pitch, while the Mesopotamian account names bitumen.
Bitumen should not be confused with clay. Clay was central to Mesopotamian construction, but it is not the waterproofing substance specified for Utnapishtim’s boat in the epic.