Harappa is the name of a site in the Pakistani part of the Punjab where an ancient city was discovered in the 1920s, much to everyone's surprise. This first find initiated the excavation of a whole system of towns and villages along the Indus, called the Harappan or the Indus Valley civilization. This is, if not the first, then one of the oldest civilizations on Earth. No one knows certainly how old Harappa and its southern sister city of Mohenjo-daro are, but they are often estimated to be from about 2500 B.C.
So the culture of Harappa was not only ancient, it was really ancient. Most impressively, at least to a reader like me, the site has revealed the oldest known examples of writing - trident-shaped markings on clay jars, believed to be either names of deities or descriptions of the contents.
The city, built on the banks of the river Indus, was flooded and rebuilt at least five times. The inhabitants kept plans of the city so that they could remake it exactly as it had been before the flood. Uncommonly for these old and venerable cultures, Harappa and the other cities do not have any great temples or palaces for gods or kings. They appear to have had a fairly equal social structure and worshipped a version of what we often call the mother goddess.
Around 1800-1700 B.C the culture disappeared with almost no trace, perhaps because the important river Sarasvati dried up and the climate in general changed. The migrating Indo-Europeans did not inherit the Harappan culture, and had to build their own civilization from scratch.