When and by whom was the Indus Valley civilization first discovered?

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:

• Prehistoric Age in India
• Prehistoric Rock Paintings