The ancient city of Harappa lies around 150 miles north of Lahore, in Pakistan. It was once one of the great centres of a civilisation with vast trade connections, which built over 100 cities, some with sophisticated sanitation systems, large-scale architecture and even a modern grid layout. But the Indus civilisation in Pakistan and India had been totally forgotten until a small seal, used for making impressions in wax or clay, led to its rediscovery. (Duration: 14 mins)