According to Genesis, Noah’s Ark was coated with pitch to make it waterproof.
In Genesis 6:14, Noah is instructed to build the Ark from “gopher wood” and cover it inside and outside with pitch. Pitch is a sticky, water-resistant substance traditionally associated with bitumen or tar. The instruction gives the vessel a practical sealing layer in addition to its wooden structure.
The exact meaning of “gopher wood” is uncertain. The Hebrew term appears only in this passage, so translators cannot identify a universally accepted tree species. By contrast, the use of pitch is much clearer: it describes a coating intended to stop water entering through gaps in the construction.
The Ark account is religious literature, and its measurements and construction details have inspired many replicas. Those modern projects use contemporary materials and engineering, not necessarily the ancient substances described in Genesis. “Pitch” is also sometimes confused with “bitumen”; the terms overlap in many translations, but the canonical answer to the wording of Genesis is pitch.