Harappa Mound AB Center with the great drain looking out over Punjab. Harappa was first excavated in 1872 by Alexander Cunningham, the original Director General of the Archaeological Survey of India. Although he named the mounds and found a seal and other implements after digging a handful of trenches, he was in search of a Buddhist city and did not realize that he had come upon a Bronze Age civilization that would push back Indian history 2,000 years. It took until 1921 before Harappa was excavated again by Daya Ram Sahni (inset shows unexcavated mound that year), but even then, it was not until the excavations at Mohenjodaro soon thereafter the great antiquity and sophistication of ancient Indus times was recognized.
Nayanjot Lahiri's book Finding Forgotten Cities: How the Indus Civilization was Discovered (Penguin India) tells this story in exciting detail.