Genesis says Noah waterproofed the Ark with pitch. Genesis 6:14 instructs him to cover the vessel with pitch inside and outside, sealing the wooden structure against water.
Pitch is a general term for a thick, waterproofing substance derived from natural materials such as bitumen or tar. Ancient Near Eastern boat builders used similar substances to protect wooden boats and reed vessels from water, so the detail fits known construction practices of the region.
The biblical text distinguishes this waterproofing material from the Ark’s wood. It names gopher wood for construction and pitch for sealing. Later translations may use related terms such as bitumen or tar, but “pitch” is the traditional English answer associated with the wording of Genesis.