In the Genesis account, Noah used pitch to waterproof the Ark. Genesis 6:14 instructs him to cover the vessel inside and outside with pitch after building it from gopher wood.
Pitch is a sticky, waterproofing substance traditionally associated with tar or bitumen. Ancient boatbuilders used comparable materials to seal seams and reduce the penetration of water. The biblical term therefore fits the practical purpose described in the construction instructions, even though the precise ancient substance is not established with complete certainty.
The wording is notable because it calls for coating both the interior and exterior. Genesis does not provide a detailed engineering description of the Ark’s joints, tools, or construction process. It does, however, include enough material details to portray the vessel as sealed and compartmentalized.
A frequent mix-up is to answer “gopher wood,” which is the construction material rather than the waterproofing substance. Another is to substitute modern fiberglass or paint. The text’s own term is pitch, and that is the canonical answer for how the Ark was sealed.