Genesis says Noah covered the Ark with pitch to make it watertight.
Genesis 6:14 instructs Noah to apply pitch inside and outside the vessel. The Hebrew wording is related to terms for covering or coating, and English translations commonly use “pitch,” while some specify bitumen or another tar-like waterproofing material. The exact ancient substance cannot be identified with complete certainty from the passage alone.
Waterproof coatings were important in ancient boatbuilding. Bitumen and related materials were used in parts of the ancient Near East to seal boats and containers, so the instruction fits the technological setting of the story. The account does not describe a modern marine sealant or give a recipe. Its practical purpose is clear, however: the coating protects the wooden structure from water during the Flood.