Genesis says Noah used pitch to make the Ark watertight. In Genesis 6:14, God instructs him to cover the Ark inside and outside with pitch after building it from gopher wood.
Pitch is a generic term for a sticky, waterproofing substance derived historically from tar, bitumen, or resin. Its use fits the practical requirements of a large wooden vessel: sealing joints and reducing the flow of water through the hull. The Hebrew word translated as “pitch” is related to a root meaning to cover or smear.
The passage does not identify the exact chemical composition of Noah's pitch or explain how the material was produced. Nor does it settle what “gopher wood” means; that Hebrew term appears only in this passage and has no universally accepted botanical identification. Consequently, pitch is more certain than any proposed species of construction timber.