In 1912 Harappan seals with unknown symbols were discovered by J.Fleet. This triggered an expedition by Sir John Hubert Marshall in 1921-1922. This expedition led to the discovery of the ancient city of Harappa, thus confirming the existence of the Indus Valley civilization.
For further reading check the following articles:
|Major Sites in Indus Valley Civilization||Indus Valley Civilization|
|100 Must-Know Facts about IVC for UPSC||UPSC FAQ|
|NCERT Notes: Arts of the Indus Valley||NCERT notes for History (Modern, Medieval, Ancient), Art and Culture, and Geography|