The civilization of the Indus Valley was completely unknown until the end of the 19th century - the ruins of the city remained hidden from the world for about 3,700 years.

An excerpt from the article 26 facts about Mohenjo-daro
This was due to the specific cultural conditions under which the Hindus learned about the origins of their native tradition from the collection of ancient Sanskrit hymns of the oldest of Vedas (14th century B.C.) - Rigveda. This book considers the arrival of the Proto-Indo-European Aryans to the subcontinent as the beginning of the historical period, not earlier than in the 13th century B.C., after the fall of the Mohenjo-daro culture.