Bitumen sealed Utnapishtim’s ark in the Epic of Gilgamesh. In the Flood account, the vessel is coated with bitumen, a naturally occurring petroleum-based substance used in the ancient Near East as waterproofing.
Bitumen was valuable for sealing boats, buildings, baskets, and containers because it could make surfaces resistant to water. Its use in the Gilgamesh story fits the practical problem facing a large wooden vessel, even though the epic is a mythic narrative rather than a modern construction report.
The detail also helps distinguish the Mesopotamian account from Genesis. Genesis says Noah was instructed to cover the ark with “pitch,” a term commonly understood as a waterproofing substance, but the two texts use their own traditions and vocabulary. The Epic of Gilgamesh explicitly names bitumen in its description.
Bitumen is sometimes loosely called tar or asphalt in modern summaries. Those terms can describe related substances, but bitumen is the specific answer associated with Utnapishtim’s ark in standard translations and reference works.