According to Genesis, Noah sealed the Ark with pitch inside and out.
Genesis 6:14 tells Noah to build the Ark and cover it with pitch on the inside and outside. Pitch is a waterproofing substance, commonly associated with tar-like bitumen or another sticky protective coating. The verse presents it as a practical treatment for making the vessel watertight, distinct from the wood used for its structure.
The exact ancient substance intended by the Hebrew term is discussed by translators, and English Bibles may use words such as pitch, bitumen, or tar. “Pitch” is the conventional concise answer in English-language trivia because it directly matches many translations.
This detail is easy to confuse with the Ark's building material. Genesis separately mentions the wood for construction and the pitch for sealing. The earlier question about wood concerns the structure; this question concerns waterproofing the completed vessel.