Genesis says Noah sealed the Ark with pitch inside and outside.
The instruction appears in Genesis 6:14, where the vessel is to be made of wood and covered with pitch. Pitch is a waterproofing substance traditionally produced from bitumen, tar, or related materials, although the exact ancient product intended by the Hebrew term is debated.
This detail explains the Ark’s basic watertight construction in the narrative. It is sometimes described loosely as tar, but “pitch” is the conventional translation and is the shortest canonical answer. The passage gives no recipe, quantity, or archaeological specification for the coating.